diff --git a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldValueValidator.php b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldValueValidator.php
index c6849264b4..48d2f40526 100644
--- a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldValueValidator.php
+++ b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldValueValidator.php
@@ -24,8 +24,12 @@ public function validate($items, Constraint $constraint) {
     $id_key = $entity->getEntityType()->getKey('id');
 
     $query = \Drupal::entityQuery($entity_type_id);
-    if (!empty($entity->id())) {
-      $query->condition($id_key, $items->getEntity()->id(), '<>');
+
+    $entity_id = $entity->id();
+    // Using isset() instead of !empty() as 0 and '0' are valid ID values for
+    // entity types using string IDs.
+    if (isset($entity_id)) {
+      $query->condition($id_key, $entity_id, '<>');
     }
 
     $value_taken = (bool) $query
@@ -38,7 +42,7 @@ public function validate($items, Constraint $constraint) {
       $this->context->addViolation($constraint->message, [
         '%value' => $item->value,
         '@entity_type' => $entity->getEntityType()->getLowercaseLabel(),
-        '@field_name' => mb_strtolower($items->getFieldDefinition()->getLabel()),
+        '@field_name' => $items->getFieldDefinition()->getLabel(),
       ]);
     }
   }
diff --git a/core/modules/system/tests/modules/unique_field_constraint_test/unique_field_constraint_test.info.yml b/core/modules/system/tests/modules/unique_field_constraint_test/unique_field_constraint_test.info.yml
new file mode 100644
index 0000000000..a667c6217d
--- /dev/null
+++ b/core/modules/system/tests/modules/unique_field_constraint_test/unique_field_constraint_test.info.yml
@@ -0,0 +1,6 @@
+name: 'UniqueField Constraint Test'
+type: module
+description: 'Support module for UniqueField Constraint testing.'
+package: Testing
+version: VERSION
+core: 8.x
diff --git a/core/modules/system/tests/modules/unique_field_constraint_test/unique_field_constraint_test.module b/core/modules/system/tests/modules/unique_field_constraint_test/unique_field_constraint_test.module
new file mode 100644
index 0000000000..c9bb6ca484
--- /dev/null
+++ b/core/modules/system/tests/modules/unique_field_constraint_test/unique_field_constraint_test.module
@@ -0,0 +1,18 @@
+<?php
+
+/**
+ * @file
+ * Contains unique_field_constraint_test.module
+ */
+
+use Drupal\Core\Entity\EntityTypeInterface;
+
+/**
+ * Implements hook_entity_base_field_info_alter().
+ */
+function unique_field_constraint_test_entity_base_field_info_alter(&$fields, EntityTypeInterface $entity_type) {
+  if ($entity_type->id() === 'entity_test_string_id') {
+    /** @var \Drupal\Core\Field\BaseFieldDefinition[] $fields */
+    $fields['name']->addConstraint('UniqueField');
+  }
+}
diff --git a/core/tests/Drupal/KernelTests/Core/Validation/UniqueFieldConstraintTest.php b/core/tests/Drupal/KernelTests/Core/Validation/UniqueFieldConstraintTest.php
new file mode 100644
index 0000000000..bd4333b001
--- /dev/null
+++ b/core/tests/Drupal/KernelTests/Core/Validation/UniqueFieldConstraintTest.php
@@ -0,0 +1,115 @@
+<?php
+
+namespace Drupal\KernelTests\Core\Validation;
+
+use Drupal\Component\Render\FormattableMarkup;
+use Drupal\entity_test\Entity\EntityTestStringId;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests the unique field value validation constraint.
+ *
+ * @coversDefaultClass \Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldValueValidator
+ *
+ * @group Validation
+ */
+class UniqueFieldConstraintTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'entity_test',
+    'unique_field_constraint_test',
+    'user',
+  ];
+
+  /**
+   * Tests cases where the validation passes for entities with string IDs.
+   *
+   * @covers ::validate
+   */
+  public function testEntityWithStringId() {
+    $this->installEntitySchema('entity_test_string_id');
+
+    EntityTestStringId::create([
+      'id' => 'foo',
+      'name' => $this->randomString(),
+    ])->save();
+
+    // Reload the entity.
+    $entity = EntityTestStringId::load('foo');
+
+    // Check that an existing entity validates when the value is preserved.
+    $violations = $entity->name->validate();
+    $this->assertCount(0, $violations);
+
+    // Create a new entity with a different ID and a different field value.
+    EntityTestStringId::create([
+      'id' => 'bar',
+      'name' => $this->randomString(),
+    ]);
+
+    // Check that a new entity with a different field value validates.
+    $violations = $entity->name->validate();
+    $this->assertCount(0, $violations);
+  }
+
+  /**
+   * Tests cases when validation raises violations for entities with string IDs.
+   *
+   * @param string|int|null $id
+   *   The entity ID.
+   *
+   * @covers ::validate
+   *
+   * @dataProvider providerTestEntityWithStringIdWithViolation
+   */
+  public function testEntityWithStringIdWithViolation($id) {
+    $this->installEntitySchema('entity_test_string_id');
+
+    $value = $this->randomString();
+
+    EntityTestStringId::create([
+      'id' => 'first_entity',
+      'name' => $value,
+    ])->save();
+
+    $entity = EntityTestStringId::create([
+      'id' => $id,
+      'name' => $value,
+    ]);
+    /** @var \Symfony\Component\Validator\ConstraintViolationList $violations */
+    $violations = $entity->get('name')->validate();
+
+    $message = new FormattableMarkup('A @entity_type with @field_name %value already exists.', [
+      '%value' => $value,
+      '@entity_type' => $entity->getEntityType()->getLowercaseLabel(),
+      '@field_name' => 'Name',
+    ]);
+
+    // Check that the validation has created the appropriate violation.
+    $this->assertCount(1, $violations);
+    $this->assertEquals($message, $violations[0]->getMessage());
+  }
+
+  /**
+   * Data provider for ::testEntityWithStringIdWithViolation().
+   *
+   * @return array
+   *   An array of test cases.
+   *
+   * @see self::testEntityWithStringIdWithViolation()
+   */
+  public function providerTestEntityWithStringIdWithViolation() {
+    return [
+      'without an id' => [NULL],
+      'zero as integer' => [0],
+      'zero as string' => ["0"],
+      'non-zero as integer' => [mt_rand(1, 127)],
+      'non-zero as string' => [(string) mt_rand(1, 127)],
+      'alphanumeric' => [$this->randomMachineName()],
+    ];
+  }
+
+}
diff --git a/core/tests/Drupal/KernelTests/Core/Validation/UniqueFieldValueValidationTest.php b/core/tests/Drupal/KernelTests/Core/Validation/UniqueFieldValueValidationTest.php
deleted file mode 100644
index 93ad41bf40..0000000000
--- a/core/tests/Drupal/KernelTests/Core/Validation/UniqueFieldValueValidationTest.php
+++ /dev/null
@@ -1,125 +0,0 @@
-<?php
-
-namespace Drupal\Tests\field\Kernel;
-
-use Drupal\Core\StringTranslation\StringTranslationTrait;
-use Drupal\entity_test\Entity\EntityTestStringId;
-use Drupal\field\Entity\FieldConfig;
-use Drupal\field\Entity\FieldStorageConfig;
-
-/**
- * Tests the unique field value validation constraint.
- *
- * @group field
- */
-class UniqueFieldValueValidationTest extends FieldKernelTestBase {
-
-  use StringTranslationTrait;
-
-  /**
-   * @var string
-   */
-  private $entityType;
-
-  /**
-   * @var string
-   */
-  private $fieldName;
-
-  /**
-   * @var string
-   */
-  private $value;
-
-  /**
-   * {@inheritdoc}
-   */
-  public static $modules = ['field', 'node', 'user'];
-
-  protected function setUp() {
-    parent::setUp();
-    $this->installEntitySchema('entity_test_string_id');
-
-    $this->entityType = 'entity_test';
-    $this->fieldName = $field_storage = mb_strtolower($this->randomMachineName() . '_field_name');
-    $storage = FieldStorageConfig::create([
-      'field_name' => $this->fieldName,
-      'entity_type' => 'entity_test_string_id',
-      'type' => 'string',
-      'constraints' => [
-        'UniqueField' => [],
-      ],
-    ]);
-    $storage->save();
-
-    $field_definition = [
-      'field_storage' => $storage,
-      'bundle' => 'entity_test_string_id',
-      'field_name' => $this->fieldName,
-      'label' => $this->randomMachineName() . '_label',
-      'description' => $this->randomMachineName() . '_description',
-    ];
-
-    FieldConfig::create($field_definition)->save();
-    $this->value = $this->randomString();
-
-    EntityTestStringId::create([
-      'id' => $this->randomMachineName(),
-      $this->fieldName => $this->value,
-    ])->save();
-  }
-
-  /**
-   * Tests that the field passes validation if the value is not unique.
-   */
-  public function testNonUniqueFieldValue() {
-    $entity = EntityTestStringId::create([
-      'id' => $this->randomMachineName(),
-      $this->fieldName => $this->randomString(),
-    ]);
-    $entity->{$this->fieldName}->getFieldDefinition()->addConstraint('UniqueField');
-    $violations = $entity->{$this->fieldName}->validate();
-
-    $this->assertEquals(0, count($violations));
-  }
-
-  /**
-   * Tests that the number of values is validated against the field cardinality.
-   *
-   * @dataProvider providerTestUniqueFieldValueConstraint
-   */
-  public function testUniqueFieldValueConstraint($id) {
-    $entity = EntityTestStringId::create([
-      'id' => $id,
-      $this->fieldName => $this->value,
-    ]);
-    $entity->{$this->fieldName}->getFieldDefinition()->addConstraint('UniqueField');
-    $violations = $entity->{$this->fieldName}->validate();
-
-    $message = $this->t('A @entity_type with @field_name %value already exists.', [
-      '%value' => $this->value,
-      '@entity_type' => $entity->getEntityType()->getLowercaseLabel(),
-      '@field_name' => $entity->{$this->fieldName}->getFieldDefinition()->label(),
-    ]);
-
-    $this->assertEquals(1, count($violations));
-    $this->assertEquals($message, $violations[0]->getMessage());
-  }
-
-  /**
-   * Data provider for providerTestUniqueFieldValueConstraint.
-   *
-   * @return array
-   *   An array of test cases.
-   */
-  public function providerTestUniqueFieldValueConstraint() {
-    return [
-      'numeric id entity without an id' => [NULL],
-      'numeric id entity with an id of 0' => [0],
-      'numeric id entity with a random id' => [mt_rand(1, 127)],
-      'string id entity with empty id' => [NULL],
-      'string id entity with a random id' => [$this->randomMachineName()],
-    ];
-  }
-
-}