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()],
+    ];
+  }
+
+}