diff --git a/migrations/DoctrineMigrations/Version20260602000000.php b/migrations/DoctrineMigrations/Version20260602000000.php
new file mode 100644
index 000000000..b619d8139
--- /dev/null
+++ b/migrations/DoctrineMigrations/Version20260602000000.php
@@ -0,0 +1,77 @@
+<?php
+
+/**
+ * Copyright 2026 SURFnet B.V.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+declare(strict_types=1);
+
+namespace OpenConext\EngineBlock\Doctrine\Migrations;
+
+use Doctrine\DBAL\Schema\Index;
+use Doctrine\DBAL\Schema\Name\Identifier;
+use Doctrine\DBAL\Schema\Name\OptionallyQualifiedName;
+use Doctrine\DBAL\Schema\Name\UnqualifiedName;
+use Doctrine\DBAL\Schema\Name\UnquotedIdentifierFolding;
+use Doctrine\DBAL\Schema\Schema;
+
+final class Version20260602000000 extends AbstractEngineBlockMigration
+{
+    public function getDescription(): string
+    {
+        return 'Replace non-unique idx_user_uuid with unique index uq_user_uuid on user.uuid';
+    }
+
+    public function preUp(Schema $schema): void
+    {
+        parent::preUp($schema);
+
+        $indexes = $this->connection->createSchemaManager()->introspectTableIndexes(
+            new OptionallyQualifiedName(Identifier::unquoted('user'), null)
+        );
+        $existingIndex = array_filter(
+            $indexes,
+            static fn(Index $index) => $index->getObjectName()->equals(
+                UnqualifiedName::unquoted('idx_user_uuid'),
+                UnquotedIdentifierFolding::NONE
+            )
+        );
+
+        $this->skipIf(
+            count($existingIndex) === 0,
+            'Index idx_user_uuid on user table does not exist. Skipping (already migrated).'
+        );
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('SET SESSION innodb_sort_buffer_size = 268435456');
+        $this->addSql(
+            'ALTER TABLE `user` ADD UNIQUE INDEX `uq_user_uuid` (`uuid`), DROP INDEX `idx_user_uuid`, ALGORITHM=INPLACE, LOCK=NONE'
+        );
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql(
+            'ALTER TABLE `user` ADD INDEX `idx_user_uuid` (`uuid`), DROP INDEX `uq_user_uuid`'
+        );
+    }
+
+    public function isTransactional(): bool
+    {
+        return false;
+    }
+}
diff --git a/src/OpenConext/EngineBlockBundle/Authentication/Entity/User.php b/src/OpenConext/EngineBlockBundle/Authentication/Entity/User.php
index 747a5d97d..d5b6ff76a 100644
--- a/src/OpenConext/EngineBlockBundle/Authentication/Entity/User.php
+++ b/src/OpenConext/EngineBlockBundle/Authentication/Entity/User.php
@@ -23,7 +23,7 @@
 use OpenConext\EngineBlockBundle\Authentication\Repository\UserRepository;
 
 #[ORM\Entity(repositoryClass: UserRepository::class)]
-#[ORM\Index(name: 'idx_user_uuid', columns: ['uuid'])]
+#[ORM\UniqueConstraint(name: 'uq_user_uuid', columns: ['uuid'])]
 class User
 {
     /**