diff -u b/core/lib/Drupal/Core/Path/AliasStorage.php b/core/lib/Drupal/Core/Path/AliasStorage.php
--- b/core/lib/Drupal/Core/Path/AliasStorage.php
+++ b/core/lib/Drupal/Core/Path/AliasStorage.php
@@ -15,8 +15,9 @@
 /**
  * Provides a class for CRUD operations on path aliases.
  *
- * All queries use case-insensitive implementations, so '/test-alias' and
- * '/test-Alias' is treated as equal.
+ * All queries perform case-insensitive matching on the 'source' and 'alias'
+ * fields, so the the aliases '/test-alias' and '/test-Alias' are considered
+ * to be the same, and will both refer to the same internal system path.
  */
 class AliasStorage implements AliasStorageInterface {
   /**
@@ -101,8 +102,13 @@
   public function load($conditions) {
     $select = $this->connection->select('url_alias');
     foreach ($conditions as $field => $value) {
-      // Use LIKE for case-insensitive matching.
-      $select->condition($field, $this->connection->escapeLike($value), 'LIKE');
+      if ($field == 'source' || $field == 'alias') {
+        // Use LIKE for case-insensitive matching.
+        $select->condition($field, $this->connection->escapeLike($value), 'LIKE');
+      }
+      else {
+        $select->condition($field, $value);
+      }
     }
     return $select
       ->fields('url_alias')
@@ -119,8 +125,13 @@
     $path = $this->load($conditions);
     $query = $this->connection->delete('url_alias');
     foreach ($conditions as $field => $value) {
-      // Use LIKE for case-insensitive matching.
-      $query->condition($field, $this->connection->escapeLike($value), 'LIKE');
+      if ($field == 'source' || $field == 'alias') {
+        // Use LIKE for case-insensitive matching.
+        $query->condition($field, $this->connection->escapeLike($value), 'LIKE');
+      }
+      else {
+        $query->condition($field, $value);
+      }
     }
     $deleted = $query->execute();
     // @todo Switch to using an event for this instead of a hook.
@@ -164,21 +175,38 @@
    * {@inheritdoc}
    */
   public function lookupPathAlias($path, $langcode) {
-    $args = array(
-      ':source' => $this->connection->escapeLike($path),
-      ':langcode' => $langcode,
-      ':langcode_undetermined' => LanguageInterface::LANGCODE_NOT_SPECIFIED,
-    );
+    $source = $this->connection->escapeLike($path);
+    $langcode_undetermined = LanguageInterface::LANGCODE_NOT_SPECIFIED;
+
     // See the queries above. Use LIKE for case-insensitive matching.
     if ($langcode == LanguageInterface::LANGCODE_NOT_SPECIFIED) {
-      unset($args[':langcode']);
-      $alias = $this->connection->query("SELECT alias FROM {url_alias} WHERE source LIKE :source AND langcode = :langcode_undetermined ORDER BY pid DESC", $args)->fetchField();
+      $alias = $this->connection->select('url_alias')
+        ->fields('url_alias', ['alias'])
+        ->condition('source', $source, 'LIKE')
+        ->condition('langcode', $langcode_undetermined)
+        ->orderBy('pid', 'DESC')
+        ->execute()
+        ->fetchField();
     }
     elseif ($langcode > LanguageInterface::LANGCODE_NOT_SPECIFIED) {
-      $alias = $this->connection->query("SELECT alias FROM {url_alias} WHERE source LIKE :source AND langcode IN (:langcode, :langcode_undetermined) ORDER BY langcode DESC, pid DESC", $args)->fetchField();
+      $alias = $this->connection->select('url_alias')
+        ->fields('url_alias', ['alias'])
+        ->condition('source', $source, 'LIKE')
+        ->condition('langcode', [$langcode, $langcode_undetermined], 'IN')
+        ->orderBy('langcode', 'DESC')
+        ->orderBy('pid', 'DESC')
+        ->execute()
+        ->fetchField();
     }
     else {
-      $alias = $this->connection->query("SELECT alias FROM {url_alias} WHERE source LIKE :source AND langcode IN (:langcode, :langcode_undetermined) ORDER BY langcode ASC, pid DESC", $args)->fetchField();
+      $alias = $this->connection->select('url_alias')
+        ->fields('url_alias', ['alias'])
+        ->condition('source', $source, 'LIKE')
+        ->condition('langcode', [$langcode, $langcode_undetermined], 'IN')
+        ->orderBy('langcode', 'ASC')
+        ->orderBy('pid', 'DESC')
+        ->execute()
+        ->fetchField();
     }
 
     return $alias;
@@ -188,21 +216,35 @@
    * {@inheritdoc}
    */
   public function lookupPathSource($path, $langcode) {
-    $args = array(
-      ':alias' => $this->connection->escapeLike($path),
-      ':langcode' => $langcode,
-      ':langcode_undetermined' => LanguageInterface::LANGCODE_NOT_SPECIFIED,
-    );
+    $alias = $this->connection->escapeLike($path);
+    $langcode_undetermined = LanguageInterface::LANGCODE_NOT_SPECIFIED;
+
     // See the queries above. Use LIKE for case-insensitive matching.
     if ($langcode == LanguageInterface::LANGCODE_NOT_SPECIFIED) {
-      unset($args[':langcode']);
-      $result = $this->connection->query("SELECT source FROM {url_alias} WHERE alias LIKE :alias AND langcode = :langcode_undetermined ORDER BY pid DESC", $args);
+      $result = $this->connection->select('url_alias')
+        ->fields('url_alias', ['source'])
+        ->condition('alias', $alias, 'LIKE')
+        ->condition('langcode', $langcode_undetermined)
+        ->orderBy('pid', 'DESC')
+        ->execute();
     }
     elseif ($langcode > LanguageInterface::LANGCODE_NOT_SPECIFIED) {
-      $result = $this->connection->query("SELECT source FROM {url_alias} WHERE alias LIKE :alias AND langcode IN (:langcode, :langcode_undetermined) ORDER BY langcode DESC, pid DESC", $args);
+      $result = $this->connection->select('url_alias')
+        ->fields('url_alias', ['source'])
+        ->condition('alias', $alias, 'LIKE')
+        ->condition('langcode', [$langcode, $langcode_undetermined], 'IN')
+        ->orderBy('langcode', 'DESC')
+        ->orderBy('pid', 'DESC')
+        ->execute();
     }
     else {
-      $result = $this->connection->query("SELECT source FROM {url_alias} WHERE alias LIKE :alias AND langcode IN (:langcode, :langcode_undetermined) ORDER BY langcode ASC, pid DESC", $args);
+      $result = $this->connection->select('url_alias')
+        ->fields('url_alias', ['source'])
+        ->condition('alias', $alias, 'LIKE')
+        ->condition('langcode', [$langcode, $langcode_undetermined], 'IN')
+        ->orderBy('langcode', 'ASC')
+        ->orderBy('pid', 'DESC')
+        ->execute();
     }
 
     return $result->fetchField();
diff -u b/core/lib/Drupal/Core/Path/AliasStorageInterface.php b/core/lib/Drupal/Core/Path/AliasStorageInterface.php
--- b/core/lib/Drupal/Core/Path/AliasStorageInterface.php
+++ b/core/lib/Drupal/Core/Path/AliasStorageInterface.php
@@ -44,7 +44,8 @@
   /**
    * Fetches a specific URL alias from the database.
    *
-   * The default implementation provides a case-insensitive implementation.
+   * The default implementation performs case-insensitive matching on the
+   * 'source' and 'alias' strings.
    *
    * @param array $conditions
    *   An array of query conditions.
@@ -62,7 +63,8 @@
   /**
    * Deletes a URL alias.
    *
-   * The default implementation provides a case-insensitive implementation.
+   * The default implementation performs case-insensitive matching on the
+   * 'source' and 'alias' strings.
    *
    * @param array $conditions
    *   An array of criteria.
@@ -86,7 +88,8 @@
   /**
    * Returns an alias of Drupal system URL.
    *
-   * The default implementation provides a case-insensitive implementation.
+   * The default implementation performs case-insensitive matching on the
+   * 'source' and 'alias' strings.
    *
    * @param string $path
    *   The path to investigate for corresponding path aliases.
@@ -102,7 +105,8 @@
   /**
    * Returns Drupal system URL of an alias.
    *
-   * The default implementation provides a case-insensitive implementation.
+   * The default implementation performs case-insensitive matching on the
+   * 'source' and 'alias' strings.
    *
    * @param string $path
    *   The path to investigate for corresponding system URLs.
@@ -118,7 +122,8 @@
   /**
    * Checks if alias already exists.
    *
-   * The default implementation provides a case-insensitive implementation.
+   * The default implementation performs case-insensitive matching on the
+   * 'source' and 'alias' strings.
    *
    * @param string $alias
    *   Alias to check against.
@@ -146,8 +151,8 @@
    * @param array $header
    *   Table header.
    * @param string|null $keys
-   *   (optional) Search keyword that may include one or more '*' as a wildcard
-   *   value.
+   *   (optional) Search keyword that may include one or more '*' as wildcard
+   *   values.
    *
    * @return array
    *   Array of items to be displayed on the current page.
diff -u b/core/modules/path/src/Form/PathFormBase.php b/core/modules/path/src/Form/PathFormBase.php
--- b/core/modules/path/src/Form/PathFormBase.php
+++ b/core/modules/path/src/Form/PathFormBase.php
@@ -182,10 +182,16 @@
     if ($this->aliasStorage->aliasExists($alias, $langcode, $this->path['source'])) {
       $stored_alias = $this->aliasStorage->load(['alias' => $alias, 'langcode' => $langcode]);
       if ($stored_alias['alias'] !== $alias) {
-        $form_state->setErrorByName('alias', t('There is an alias with other case-sensitivity: %actual_alias is already in use in this language.', ['%actual_alias' => $stored_alias['alias']]));
+        // The alias already exists with different capitalization as the default
+        // implementation of AliasStorageInterface::aliasExists is
+        // case-insensitive.
+        $form_state->setErrorByName('alias', t('The alias %alias could not be added because it is already in use in this language with different capitalization: %stored_alias.', [
+          '%alias' => $alias,
+          '%stored_alias' => $stored_alias['alias'],
+        ]));
       }
       else {
-        $form_state->setErrorByName('alias', t('The alias %alias is already in use in this language.', array('%alias' => $alias)));
+        $form_state->setErrorByName('alias', t('The alias %alias is already in use in this language.', ['%alias' => $alias]));
       }
     }
 
diff -u b/core/modules/path/src/Tests/PathAliasTest.php b/core/modules/path/src/Tests/PathAliasTest.php
--- b/core/modules/path/src/Tests/PathAliasTest.php
+++ b/core/modules/path/src/Tests/PathAliasTest.php
@@ -9,6 +9,8 @@
 
 use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Cache\Cache;
+use Drupal\simpletest\RandomGeneratorTrait;
+use Drupal\Core\Database\Database;
 
 /**
  * Add, edit, delete, and change alias and verify its consistency in the
@@ -18,6 +20,8 @@
  */
 class PathAliasTest extends PathTestBase {
 
+  use RandomGeneratorTrait;
+
   /**
    * Modules to enable.
    *
@@ -76,7 +80,7 @@
     // Create alias.
     $edit = array();
     $edit['source'] = '/node/' . $node1->id();
-    $edit['alias'] = '/' . $this->randomMachineName(8);
+    $edit['alias'] = '/' . $this->getRandomGenerator()->word(8);
     $this->drupalPostForm('admin/config/search/path/add', $edit, t('Save'));
 
     // Confirm that the alias works.
@@ -84,7 +88,8 @@
     $this->assertText($node1->label(), 'Alias works.');
     $this->assertResponse(200);
     // Confirm that the alias works in a case-insensitive way.
-    $this->drupalGet(Unicode::strtolower($edit['alias']));
+    $this->assertTrue(ctype_lower(ltrim($edit['alias'], '/')));
+    $this->drupalGet($edit['alias']);
     $this->assertText($node1->label(), 'Alias works lower case.');
     $this->assertResponse(200);
     $this->drupalGet(Unicode::strtoupper($edit['alias']));
@@ -95,13 +100,20 @@
     $pid = $this->getPID($edit['alias']);
 
     $previous = $edit['alias'];
-    $edit['alias'] = "/- ._~!$'\"()*@[]?&+%#,;=:" . // "Special" ASCII characters.
+    $edit['alias'] = '/alias' . // Lower-case letters.
+      "- ._~!$'\"()*@[]?&+%#,;=:" . // "Special" ASCII characters.
       "%23%25%26%2B%2F%3F" . // Characters that look like a percent-escaped string.
-      "éøïвβ中國書۞"; // Characters from various non-ASCII alphabets.
+      "中國書۞"; // Characters from various non-ASCII alphabets.
+      $connection = Database::getConnection();
+      if ($connection->databaseType() != 'sqlite') {
+        // When using LIKE for case-insensitivity, the SQLite driver is
+        // currently unable to find the upper-case versions of these characters.
+        $edit['alias'] .= "ïвβéø"; // Lower-case characters from various non-ASCII alphabets.
+      }
     $this->drupalPostForm('admin/config/search/path/edit/' . $pid, $edit, t('Save'));
 
     // Confirm that the alias works.
-    $this->drupalGet(Unicode::strtolower($edit['alias']));
+    $this->drupalGet(Unicode::strtoupper($edit['alias']));
     $this->assertText($node1->label(), 'Changed alias works.');
     $this->assertResponse(200);
 
@@ -122,6 +134,14 @@
     // Confirm no duplicate was created.
     $this->assertRaw(t('The alias %alias is already in use in this language.', array('%alias' => $edit['alias'])), 'Attempt to move alias was rejected.');
 
+    $edit_upper = $edit;
+    $edit_upper['alias'] = Unicode::strtoupper($edit['alias']);
+    $this->drupalPostForm('admin/config/search/path/add', $edit_upper, t('Save'));
+    $this->assertRaw(t('The alias %alias could not be added because it is already in use in this language with different capitalization: %stored_alias.', [
+      '%alias' => $edit_upper['alias'],
+      '%stored_alias' => $edit['alias'],
+    ]), 'Attempt to move upper-case alias was rejected.');
+
     // Delete alias.
     $this->drupalPostForm('admin/config/search/path/edit/' . $pid, array(), t('Delete'));
     $this->drupalPostForm(NULL, array(), t('Confirm'));
@@ -227,13 +247,20 @@
 
     // Change alias to one containing "exotic" characters.
     $previous = $edit['path[0][alias]'];
-    $edit['path[0][alias]'] = "/- ._~!$'\"()*@[]?&+%#,;=:" . // "Special" ASCII characters.
+    $edit['path[0][alias]'] = '/alias' . // Lower-case letters.
+      "- ._~!$'\"()*@[]?&+%#,;=:" . // "Special" ASCII characters.
       "%23%25%26%2B%2F%3F" . // Characters that look like a percent-escaped string.
-      "éøïвβ中國書۞"; // Characters from various non-ASCII alphabets.
+      "中國書۞"; // Characters from various non-ASCII alphabets.
+    $connection = Database::getConnection();
+    if ($connection->databaseType() != 'sqlite') {
+      // When using LIKE for case-insensitivity, the SQLite driver is
+      // currently unable to find the upper-case versions of these characters.
+      $edit['path[0][alias]'] .= "ïвβéø"; // Lower-case characters from various non-ASCII alphabets.
+    }
     $this->drupalPostForm('node/' . $node1->id() . '/edit', $edit, t('Save'));
 
     // Confirm that the alias works.
-    $this->drupalGet(Unicode::strtolower($edit['path[0][alias]']));
+    $this->drupalGet(Unicode::strtoupper($edit['path[0][alias]']));
     $this->assertText($node1->label(), 'Changed alias works.');
     $this->assertResponse(200);