diff --git a/core/modules/jsonapi/src/Controller/EntityResource.php b/core/modules/jsonapi/src/Controller/EntityResource.php
index 73c7f3ea10..561294f229 100644
--- a/core/modules/jsonapi/src/Controller/EntityResource.php
+++ b/core/modules/jsonapi/src/Controller/EntityResource.php
@@ -981,7 +981,7 @@ protected function buildWrappedResponse($data, Request $request, IncludedData $i
     assert($data instanceof Data || $data instanceof FieldItemListInterface);
     $links = ($links ?: new LinkCollection([]));
     if (!$links->hasLinkWithKey('self')) {
-      $self_link = new Link(new CacheableMetadata(), self::getRequestLink($request), ['self']);
+      $self_link = new Link(new CacheableMetadata(), self::getRequestLink($request), 'self');
       $links = $links->withLink('self', $self_link);
     }
     $response = new ResourceResponse(new JsonApiDocumentTopLevel($data, $includes, $links, $meta), $response_code, $headers);
@@ -1264,20 +1264,20 @@ protected static function getPagerLinks(Request $request, OffsetPage $page_param
     // Check if this is not the last page.
     if ($link_context['has_next_page']) {
       $next_url = static::getRequestLink($request, static::getPagerQueries('next', $offset, $size, $query));
-      $pager_links = $pager_links->withLink('next', new Link(new CacheableMetadata(), $next_url, ['next']));
+      $pager_links = $pager_links->withLink('next', new Link(new CacheableMetadata(), $next_url, 'next'));
 
       if (!empty($total)) {
         $last_url = static::getRequestLink($request, static::getPagerQueries('last', $offset, $size, $query, $total));
-        $pager_links = $pager_links->withLink('last', new Link(new CacheableMetadata(), $last_url, ['last']));
+        $pager_links = $pager_links->withLink('last', new Link(new CacheableMetadata(), $last_url, 'last'));
       }
     }
 
     // Check if this is not the first page.
     if ($offset > 0) {
       $first_url = static::getRequestLink($request, static::getPagerQueries('first', $offset, $size, $query));
-      $pager_links = $pager_links->withLink('first', new Link(new CacheableMetadata(), $first_url, ['first']));
+      $pager_links = $pager_links->withLink('first', new Link(new CacheableMetadata(), $first_url, 'first'));
       $prev_url = static::getRequestLink($request, static::getPagerQueries('prev', $offset, $size, $query));
-      $pager_links = $pager_links->withLink('prev', new Link(new CacheableMetadata(), $prev_url, ['prev']));
+      $pager_links = $pager_links->withLink('prev', new Link(new CacheableMetadata(), $prev_url, 'prev'));
     }
 
     return $pager_links;
diff --git a/core/modules/jsonapi/src/Controller/EntryPoint.php b/core/modules/jsonapi/src/Controller/EntryPoint.php
index bbbc2d56d1..12d5efab0c 100644
--- a/core/modules/jsonapi/src/Controller/EntryPoint.php
+++ b/core/modules/jsonapi/src/Controller/EntryPoint.php
@@ -82,7 +82,7 @@ public function index() {
       return !$resource->isInternal();
     });
 
-    $self_link = new Link(new CacheableMetadata(), Url::fromRoute('jsonapi.resource_list'), ['self']);
+    $self_link = new Link(new CacheableMetadata(), Url::fromRoute('jsonapi.resource_list'), 'self');
     $urls = array_reduce($resources, function (LinkCollection $carry, ResourceType $resource_type) {
       if ($resource_type->isLocatable() || $resource_type->isMutable()) {
         $route_suffix = $resource_type->isLocatable() ? 'collection' : 'collection.post';
diff --git a/core/modules/jsonapi/src/Controller/FileUpload.php b/core/modules/jsonapi/src/Controller/FileUpload.php
index 27032eaf3a..eb9fd6cd44 100644
--- a/core/modules/jsonapi/src/Controller/FileUpload.php
+++ b/core/modules/jsonapi/src/Controller/FileUpload.php
@@ -178,7 +178,7 @@ public function handleFileUploadForNewResource(Request $request, ResourceType $r
     }
 
     // @todo Remove line below in favor of commented line in https://www.drupal.org/project/jsonapi/issues/2878463.
-    $self_link = new Link(new CacheableMetadata(), Url::fromRoute('jsonapi.file--file.individual', ['entity' => $file->uuid()]), ['self']);
+    $self_link = new Link(new CacheableMetadata(), Url::fromRoute('jsonapi.file--file.individual', ['entity' => $file->uuid()]), 'self');
     /* $self_link = new Link(new CacheableMetadata(), $this->entity->toUrl('jsonapi'), ['self']); */
     $links = new LinkCollection(['self' => $self_link]);
 
diff --git a/core/modules/jsonapi/src/JsonApiResource/Link.php b/core/modules/jsonapi/src/JsonApiResource/Link.php
index 5634f983df..8aad002058 100644
--- a/core/modules/jsonapi/src/JsonApiResource/Link.php
+++ b/core/modules/jsonapi/src/JsonApiResource/Link.php
@@ -3,6 +3,7 @@
 namespace Drupal\jsonapi\JsonApiResource;
 
 use Drupal\Component\Assertion\Inspector;
+use Drupal\Component\Serialization\Json;
 use Drupal\Core\Cache\CacheableDependencyInterface;
 use Drupal\Core\Cache\CacheableDependencyTrait;
 use Drupal\Core\Cache\CacheableMetadata;
@@ -41,6 +42,8 @@
    * The link relation types.
    *
    * @var string[]
+   *
+   * @todo: change this type documentation to be a single string in https://www.drupal.org/project/drupal/issues/3080467.
    */
   protected $rel;
 
@@ -64,16 +67,24 @@
    *   entity on which the link will appear.
    * @param \Drupal\Core\Url $url
    *   The Url object for the link.
-   * @param string[] $link_relation_types
+   * @param string $link_relation_type
    *   An array of registered or extension RFC8288 link relation types.
    * @param array $target_attributes
    *   An associative array of target attributes for the link.
    *
    * @see https://tools.ietf.org/html/rfc8288#section-2.1
    */
-  public function __construct(CacheableMetadata $cacheability, Url $url, array $link_relation_types, array $target_attributes = []) {
+  public function __construct(CacheableMetadata $cacheability, Url $url, $link_relation_type, array $target_attributes = []) {
+    // @todo Remove this conditional block in drupal:9.0.0 and add a type hint to the $link_relation_type argument of this method in https://www.drupal.org/project/drupal/issues/3080467.
+    // @see https://www.drupal.org/project/drupal/issues/3080259
+    if (is_array($link_relation_type)) {
+      @trigger_error('Constructing a ' . self::class . ' with an array of link relation types is deprecated in drupal:8.8.0 and will throw a fatal error in drupal:9.0.0. Pass a single string instead. See https://www.drupal.org/project/drupal/issues/3080259.', E_USER_DEPRECATED);
+    }
+    else {
+      $link_relation_type = [$link_relation_type];
+    }
     // @todo: uncomment the extra assertion below when JSON:API begins to use its own extension relation types.
-    assert(/* !empty($link_relation_types) && */Inspector::assertAllStrings($link_relation_types));
+    assert(/* !empty($link_relation_types) && */Inspector::assertAllStrings($link_relation_type));
     assert(Inspector::assertAllStrings(array_keys($target_attributes)));
     assert(Inspector::assertAll(function ($target_attribute_value) {
       return is_string($target_attribute_value) || is_array($target_attribute_value);
@@ -81,7 +92,7 @@ public function __construct(CacheableMetadata $cacheability, Url $url, array $li
     $generated_url = $url->setAbsolute()->toString(TRUE);
     $this->href = $generated_url->getGeneratedUrl();
     $this->uri = $url;
-    $this->rel = $link_relation_types;
+    $this->rel = $link_relation_type;
     $this->attributes = $target_attributes;
     $this->setCacheability($cacheability->addCacheableDependency($generated_url));
   }
@@ -106,14 +117,32 @@ public function getHref() {
     return $this->href;
   }
 
+  /**
+   * Gets the link's relation types.
+   *
+   * @return string|string[]
+   *   The link's relation type. For BC reasons, this method might return an
+   *   array of strings.
+   *
+   * @todo: update the return type docs to only have a single string in https://www.drupal.org/project/drupal/issues/3080467.
+   */
+  public function getLinkRelationType() {
+    // @todo: uncomment the following line and remove the one after it in https://www.drupal.org/project/drupal/issues/3080467.
+    //return $this->rel;
+    return count($this->rel) > 1 ? $this->rel : reset($this->rel);
+  }
+
   /**
    * Gets the link's relation types.
    *
    * @return string[]
    *   The link's relation types.
+   *
+   * @todo: remove this method in https://www.drupal.org/project/drupal/issues/3080467.
    */
   public function getLinkRelationTypes() {
-    return $this->rel;
+    @trigger_error(self::class . '::' . __METHOD__ . ' is deprecated in drupal:8.8.0 and will be removed in drupal:9.0.0. Use ' . self::class . '::' . 'getLinkRelationType' . ' instead. See https://www.drupal.org/project/drupal/issues/3080259.', E_USER_DEPRECATED);
+    return (array) $this->getLinkRelationType();
   }
 
   /**
@@ -127,7 +156,7 @@ public function getTargetAttributes() {
   }
 
   /**
-   * Compares two links by their href.
+   * Compares two links.
    *
    * @param \Drupal\jsonapi\JsonApiResource\Link $a
    *   The first link.
@@ -135,16 +164,31 @@ public function getTargetAttributes() {
    *   The second link.
    *
    * @return int
-   *   The result of strcmp() on the links' hrefs.
+   *   0 if the links can be considered identical, an integer greater than or
+   *   less than 0 otherwise.
    */
   public static function compare(Link $a, Link $b) {
-    return strcmp($a->getHref(), $b->getHref());
+    // Sort the link relation type array so that order does not matter during
+    // string comparison.
+    sort($a->rel);
+    sort($b->rel);
+    // Any string concatenation would work, but a Link header-like format makes
+    // it clear what is being compared.
+    $a_string = sprintf('<%s>;rel="%s"', $a->getHref(), implode(' ', $a->rel));
+    $b_string = sprintf('<%s>;rel="%s"', $b->getHref(), implode(' ', $b->rel));
+    $cmp = strcmp($a_string, $b_string);
+    // This ternary avoids unnecessarily encoding target attributes on non-
+    // equivalent links.
+    $json_cmp = strcmp(Json::encode($a->getTargetAttributes()), Json::encode($b->getTargetAttributes()));
+    return $cmp === 0
+      ? $json_cmp
+      : $cmp;
   }
 
   /**
-   * Merges two link objects' relation types and target attributes.
+   * Merges two equivalent links into one link with the merged cacheability.
    *
-   * The links must share the same URI.
+   * The links must share the same URI, link relation type and attributes.
    *
    * @param \Drupal\jsonapi\JsonApiResource\Link $a
    *   The first link.
@@ -152,15 +196,14 @@ public static function compare(Link $a, Link $b) {
    *   The second link.
    *
    * @return static
-   *   A new JSON:API Link object with the link relation type and target
-   *   attributes merged.
+   *   A new JSON:API Link object with the cacheability of both links merged.
    */
   public static function merge(Link $a, Link $b) {
-    assert(static::compare($a, $b) === 0);
-    $merged_rels = array_unique(array_merge($a->getLinkRelationTypes(), $b->getLinkRelationTypes()));
-    $merged_attributes = array_merge_recursive($a->getTargetAttributes(), $b->getTargetAttributes());
+    if (static::compare($a, $b) !== 0) {
+      throw new \LogicException('Only equivalent links can be merged.');
+    }
     $merged_cacheability = (new CacheableMetadata())->addCacheableDependency($a)->addCacheableDependency($b);
-    return new static($merged_cacheability, $a->getUri(), $merged_rels, $merged_attributes);
+    return new static($merged_cacheability, $a->getUri(), $a->getLinkRelationType(), $a->getTargetAttributes());
   }
 
 }
diff --git a/core/modules/jsonapi/src/JsonApiResource/ResourceObject.php b/core/modules/jsonapi/src/JsonApiResource/ResourceObject.php
index 0540248c9b..f870368e86 100644
--- a/core/modules/jsonapi/src/JsonApiResource/ResourceObject.php
+++ b/core/modules/jsonapi/src/JsonApiResource/ResourceObject.php
@@ -243,19 +243,19 @@ protected static function buildLinksFromEntity(ResourceType $resource_type, Enti
           // revision changes and to disambiguate resource objects with the same
           // `type` and `id` in a `version-history` collection.
           $self_with_version_url = $self_url->setOption('query', [JsonApiSpec::VERSION_QUERY_PARAMETER => 'id:' . $entity->getRevisionId()]);
-          $links = $links->withLink('self', new Link(new CacheableMetadata(), $self_with_version_url, ['self']));
+          $links = $links->withLink('self', new Link(new CacheableMetadata(), $self_with_version_url, 'self'));
         }
         if (!$entity->isDefaultRevision()) {
           $latest_version_url = $self_url->setOption('query', [JsonApiSpec::VERSION_QUERY_PARAMETER => 'rel:' . VersionByRel::LATEST_VERSION]);
-          $links = $links->withLink(VersionByRel::LATEST_VERSION, new Link(new CacheableMetadata(), $latest_version_url, [VersionByRel::LATEST_VERSION]));
+          $links = $links->withLink(VersionByRel::LATEST_VERSION, new Link(new CacheableMetadata(), $latest_version_url, VersionByRel::LATEST_VERSION));
         }
         if (!$entity->isLatestRevision()) {
           $working_copy_url = $self_url->setOption('query', [JsonApiSpec::VERSION_QUERY_PARAMETER => 'rel:' . VersionByRel::WORKING_COPY]);
-          $links = $links->withLink(VersionByRel::WORKING_COPY, new Link(new CacheableMetadata(), $working_copy_url, [VersionByRel::WORKING_COPY]));
+          $links = $links->withLink(VersionByRel::WORKING_COPY, new Link(new CacheableMetadata(), $working_copy_url, VersionByRel::WORKING_COPY));
         }
       }
       if (!$links->hasLinkWithKey('self')) {
-        $links = $links->withLink('self', new Link(new CacheableMetadata(), $self_url, ['self']));
+        $links = $links->withLink('self', new Link(new CacheableMetadata(), $self_url, 'self'));
       }
     }
     return $links;
diff --git a/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php b/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php
index 6257adb4d8..fece326ab5 100644
--- a/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php
+++ b/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php
@@ -380,7 +380,7 @@ protected function normalize(EntityInterface $entity, Url $url) {
     // Don't use cached normalizations in tests.
     $this->container->get('cache.jsonapi_normalizations')->deleteAll();
 
-    $self_link = new Link(new CacheableMetadata(), $url, ['self']);
+    $self_link = new Link(new CacheableMetadata(), $url, 'self');
     $resource_type = $this->container->get('jsonapi.resource_type.repository')->getByTypeName(static::$resourceTypeName);
     $doc = new JsonApiDocumentTopLevel(new ResourceObjectData([ResourceObject::createFromEntity($resource_type, $entity)], 1), new NullIncludedData(), new LinkCollection(['self' => $self_link]));
     return $this->serializer->normalize($doc, 'api_json', [
diff --git a/core/modules/jsonapi/tests/src/Unit/JsonApiResource/LinkTest.php b/core/modules/jsonapi/tests/src/Unit/JsonApiResource/LinkTest.php
index 91441a8ea5..ee60421371 100644
--- a/core/modules/jsonapi/tests/src/Unit/JsonApiResource/LinkTest.php
+++ b/core/modules/jsonapi/tests/src/Unit/JsonApiResource/LinkTest.php
@@ -19,67 +19,99 @@
 class LinkTest extends UnitTestCase {
 
   /**
-   * @covers ::merge
-   * @dataProvider mergeTargetAttributesProvider
+   * @covers ::compare
+   * @dataProvider linkComparisonProvider
    */
-  public function testMergeTargetAttributes($a, $b, $expected) {
-    $this->assertSame($expected->getTargetAttributes(), Link::merge($a, $b)->getTargetAttributes());
+  public function testLinkComparison(Link $a, Link $b, $expected) {
+    $actual = Link::compare($a, $b);
+    $this->assertSame($expected, $actual === 0);
   }
 
   /**
-   * Provides test data for link merging.
+   * Provides test data for link comparison.
    */
-  public function mergeTargetAttributesProvider() {
-    $cases = [
-      'strings' => [
-        ['key' => 'foo'],
-        ['key' => 'bar'],
-        ['key' => ['foo', 'bar']],
+  public function linkComparisonProvider() {
+    $this->mockUrlAssembler();
+    return [
+      'same href and same link relation type' => [
+        new Link(new CacheableMetadata(), Url::fromUri('https://jsonapi.org/foo'), 'self'),
+        new Link(new CacheableMetadata(), Url::fromUri('https://jsonapi.org/foo'), 'self'),
+        TRUE,
+      ],
+      'different href and same link relation type' => [
+        new Link(new CacheableMetadata(), Url::fromUri('https://jsonapi.org/foo'), 'self'),
+        new Link(new CacheableMetadata(), Url::fromUri('https://jsonapi.org/bar'), 'self'),
+        FALSE,
+      ],
+      'same href and different link relation type' => [
+        new Link(new CacheableMetadata(), Url::fromUri('https://jsonapi.org/foo'), 'self'),
+        new Link(new CacheableMetadata(), Url::fromUri('https://jsonapi.org/foo'), 'related'),
+        FALSE,
       ],
-      'string and array' => [
-        ['key' => 'foo'],
-        ['key' => ['bar', 'baz']],
-        ['key' => ['foo', 'bar', 'baz']],
+      'same href and same link relation type and empty target attributes' => [
+        new Link(new CacheableMetadata(), Url::fromUri('https://jsonapi.org/foo'), 'self', []),
+        new Link(new CacheableMetadata(), Url::fromUri('https://jsonapi.org/foo'), 'self', []),
+        TRUE,
       ],
-      'one-dimensional indexed arrays' => [
-        ['key' => ['foo']],
-        ['key' => ['bar']],
-        ['key' => ['foo', 'bar']],
+      'same href and same link relation type and same target attributes' => [
+        new Link(new CacheableMetadata(), Url::fromUri('https://jsonapi.org/foo'), 'self', ['anchor' => 'https://jsonapi.org']),
+        new Link(new CacheableMetadata(), Url::fromUri('https://jsonapi.org/foo'), 'self', ['anchor' => 'https://jsonapi.org']),
+        TRUE,
       ],
-      'one-dimensional keyed arrays' => [
-        ['key' => ['foo' => 'tball']],
-        ['key' => ['bar' => 'ista']],
-        [
-          'key' => [
-            'foo' => 'tball',
-            'bar' => 'ista',
-          ],
-        ],
+      'same href and same link relation type and different target attributes' => [
+        new Link(new CacheableMetadata(), Url::fromUri('https://jsonapi.org/foo'), 'self', ['anchor' => 'https://jsonapi.org/foo']),
+        new Link(new CacheableMetadata(), Url::fromUri('https://jsonapi.org/foo'), 'self', ['anchor' => 'https://jsonapi.org/bar']),
+        FALSE,
       ],
-      'two-dimensional indexed arrays' => [
-        ['one' => ['two' => ['foo']]],
-        ['one' => ['two' => ['bar']]],
-        ['one' => ['two' => ['foo', 'bar']]],
+      'same href and same link relation type and same nested target attributes' => [
+        new link(new cacheablemetadata(), url::fromuri('https://jsonapi.org/foo'), 'self', ['data' => ['foo' => 'bar']]),
+        new link(new cacheablemetadata(), url::fromuri('https://jsonapi.org/foo'), 'self', ['data' => ['foo' => 'bar']]),
+        TRUE,
       ],
-      'two-dimensional keyed arrays' => [
-        ['one' => ['two' => ['foo' => 'tball']]],
-        ['one' => ['two' => ['bar' => 'ista']]],
-        [
-          'one' => [
-            'two' => [
-              'foo' => 'tball',
-              'bar' => 'ista',
-            ],
-          ],
-        ],
+      'same href and same link relation type and different nested target attributes' => [
+        new link(new cacheablemetadata(), url::fromuri('https://jsonapi.org/foo'), 'self', ['data' => ['foo' => 'bar']]),
+        new link(new cacheablemetadata(), url::fromuri('https://jsonapi.org/foo'), 'self', ['data' => ['foo' => 'baz']]),
+        FALSE,
       ],
     ];
+  }
+
+  /**
+   * @covers ::merge
+   * @dataProvider linkMergeProvider
+   */
+  public function testLinkMerge(Link $a, Link $b, $expected) {
+    if ($expected instanceof Link) {
+      $this->assertSame($expected->getCacheTags(), Link::merge($a, $b)->getCacheTags());
+    }
+    else {
+      $this->expectException($expected);
+      Link::merge($a, $b);
+    }
+  }
+
+  /**
+   * Provides test data for link merging.
+   */
+  public function linkMergeProvider() {
     $this->mockUrlAssembler();
-    return array_map(function ($arguments) {
-      return array_map(function ($attributes) {
-        return new Link(new CacheableMetadata(), Url::fromUri('https://jsonapi.org/'), ['item'], $attributes);
-      }, $arguments);
-    }, $cases);
+    return [
+      'same everything' => [
+        new Link((new CacheableMetadata())->addCacheTags(['foo']), Url::fromUri('https://jsonapi.org/foo'), 'self'),
+        new Link((new CacheableMetadata())->addCacheTags(['foo']), Url::fromUri('https://jsonapi.org/foo'), 'self'),
+        new Link((new CacheableMetadata())->addCacheTags(['foo']), Url::fromUri('https://jsonapi.org/foo'), 'self'),
+      ],
+      'different cache tags' => [
+        new Link((new CacheableMetadata())->addCacheTags(['foo']), Url::fromUri('https://jsonapi.org/foo'), 'self'),
+        new Link((new CacheableMetadata())->addCacheTags(['bar']), Url::fromUri('https://jsonapi.org/foo'), 'self'),
+        new Link((new CacheableMetadata())->addCacheTags(['foo', 'bar']), Url::fromUri('https://jsonapi.org/foo'), 'self'),
+      ],
+      'same cache tags, non-equivalent links' => [
+        new Link((new CacheableMetadata())->addCacheTags(['foo']), Url::fromUri('https://jsonapi.org/foo'), 'self'),
+        new Link((new CacheableMetadata())->addCacheTags(['foo']), Url::fromUri('https://jsonapi.org/bar'), 'self'),
+        \LogicException::class,
+      ],
+    ];
   }
 
   /**
@@ -89,7 +121,9 @@ protected function mockUrlAssembler() {
     $url_assembler = $this->getMockBuilder(UnroutedUrlAssemblerInterface::class)
       ->disableOriginalConstructor()
       ->getMock();
-    $url_assembler->method('assemble')->willReturn((new GeneratedUrl())->setGeneratedUrl('https://jsonapi.org/'));
+    $url_assembler->method('assemble')->will($this->returnCallback(function ($uri) {
+      return (new GeneratedUrl())->setGeneratedUrl($uri);
+    }));
 
     $container = new ContainerBuilder();
     $container->set('unrouted_url_assembler', $url_assembler);