diff --git a/app/eventyay/api/tests/__init__.py b/app/eventyay/api/tests/__init__.py
new file mode 100644
index 0000000000..fcaedc43cd
--- /dev/null
+++ b/app/eventyay/api/tests/__init__.py
@@ -0,0 +1 @@
+# API Tests
diff --git a/app/eventyay/api/tests/conftest.py b/app/eventyay/api/tests/conftest.py
new file mode 100644
index 0000000000..73dd069526
--- /dev/null
+++ b/app/eventyay/api/tests/conftest.py
@@ -0,0 +1,190 @@
+"""
+Test fixtures for API tests.
+
+These fixtures provide common test data for testing the API endpoints.
+"""
+from datetime import datetime, timezone
+from decimal import Decimal
+
+import pytest
+from django.test import utils
+from django_scopes import scopes_disabled
+from rest_framework.test import APIClient
+
+from eventyay.base.models import (
+    Device,
+    Event,
+    Order,
+    OrderFee,
+    OrderPayment,
+    OrderPosition,
+    Organizer,
+    Team,
+    User,
+)
+from eventyay.base.models.devices import generate_api_token
+
+
+@pytest.fixture
+def client():
+    """Return an API test client."""
+    return APIClient()
+
+
+@pytest.fixture
+@scopes_disabled()
+def organizer():
+    """Create a test organizer."""
+    return Organizer.objects.create(name='Test Organizer', slug='test-organizer')
+
+
+@pytest.fixture
+@scopes_disabled()
+def event(organizer):
+    """Create a test event."""
+    e = Event.objects.create(
+        organizer=organizer,
+        name='Test Event',
+        slug='test-event',
+        date_from=datetime(2025, 12, 27, 10, 0, 0, tzinfo=timezone.utc),
+        plugins='eventyay.plugins.banktransfer,eventyay.plugins.ticketoutputpdf',
+        is_public=True,
+    )
+    e.settings.timezone = 'UTC'
+    return e
+
+
+@pytest.fixture
+@scopes_disabled()
+def team(organizer):
+    """Create a test team with full permissions."""
+    return Team.objects.create(
+        organizer=organizer,
+        name='Test Team',
+        can_change_teams=True,
+        can_manage_gift_cards=True,
+        can_change_items=True,
+        can_create_events=True,
+        can_change_event_settings=True,
+        can_change_vouchers=True,
+        can_view_vouchers=True,
+        can_change_orders=True,
+        can_change_organizer_settings=True,
+    )
+
+
+@pytest.fixture
+@scopes_disabled()
+def device(organizer):
+    """Create a test API device."""
+    return Device.objects.create(
+        organizer=organizer,
+        all_events=True,
+        name='Test Device',
+        initialized=datetime.now(timezone.utc),
+        api_token=generate_api_token(),
+    )
+
+
+@pytest.fixture
+def user():
+    """Create a test user."""
+    return User.objects.create_user('test@test.test', 'testpassword')
+
+
+@pytest.fixture
+@scopes_disabled()
+def user_client(client, team, user):
+    """Return an API client authenticated as a user."""
+    team.can_view_orders = True
+    team.can_view_vouchers = True
+    team.all_events = True
+    team.save()
+    team.members.add(user)
+    client.force_authenticate(user=user)
+    return client
+
+
+@pytest.fixture
+@scopes_disabled()
+def token_client(client, team):
+    """Return an API client authenticated with a team token."""
+    team.can_view_orders = True
+    team.can_change_orders = True
+    team.can_view_vouchers = True
+    team.all_events = True
+    team.save()
+    t = team.tokens.create(name='Test Token')
+    client.credentials(HTTP_AUTHORIZATION='Token ' + t.token)
+    return client
+
+
+@pytest.fixture
+def device_client(client, device):
+    """Return an API client authenticated as a device."""
+    client.credentials(HTTP_AUTHORIZATION='Device ' + device.api_token)
+    return client
+
+
+@pytest.fixture
+@scopes_disabled()
+def item(event):
+    """Create a test product/item."""
+    return event.products.create(name='Test Ticket', default_price=Decimal('100.00'))
+
+
+@pytest.fixture
+@scopes_disabled()
+def taxrule(event):
+    """Create a test tax rule."""
+    return event.tax_rules.create(name='VAT', rate=Decimal('19.00'))
+
+
+@pytest.fixture
+@scopes_disabled()
+def order(event, item, taxrule):
+    """Create a test order with a payment."""
+    o = Order.objects.create(
+        code='TEST01',
+        event=event,
+        email='customer@test.test',
+        status=Order.STATUS_PENDING,
+        secret='testsecretkey12345',
+        datetime=datetime(2025, 1, 1, 10, 0, 0, tzinfo=timezone.utc),
+        expires=datetime(2025, 1, 10, 10, 0, 0, tzinfo=timezone.utc),
+        total=Decimal('100.00'),
+        locale='en',
+    )
+    
+    # Create a confirmed payment
+    o.payments.create(
+        provider='manual',
+        state=OrderPayment.PAYMENT_STATE_CONFIRMED,
+        amount=Decimal('100.00'),
+        payment_date=datetime(2025, 1, 1, 10, 0, 0, tzinfo=timezone.utc),
+    )
+    
+    # Create a fee
+    o.fees.create(
+        fee_type=OrderFee.FEE_TYPE_PAYMENT,
+        value=Decimal('0.00'),
+        tax_rate=Decimal('0.00'),
+        tax_value=Decimal('0.00'),
+    )
+    
+    # Create an order position
+    OrderPosition.objects.create(
+        order=o,
+        product=item,
+        variation=None,
+        price=Decimal('100.00'),
+        attendee_name_parts={'full_name': 'Test Customer', '_scheme': 'full'},
+        secret='testpositionsecret123',
+        pseudonymization_id='TESTPSEUDO',
+    )
+    
+    return o
+
+
+# Apply scopes_disabled to database setup
+utils.setup_databases = scopes_disabled()(utils.setup_databases)
diff --git a/app/eventyay/api/tests/test_concurrent_refunds.py b/app/eventyay/api/tests/test_concurrent_refunds.py
new file mode 100644
index 0000000000..905f544d2e
--- /dev/null
+++ b/app/eventyay/api/tests/test_concurrent_refunds.py
@@ -0,0 +1,104 @@
+"""
+Test for concurrent refund race condition prevention.
+
+Validates that the select_for_update() fix in OrderPaymentViewSet.refund()
+prevents multiple concurrent refunds from exceeding the payment amount.
+
+Related: app/eventyay/api/views/order.py lines 1251-1276
+"""
+import json
+import threading
+from decimal import Decimal
+
+import pytest
+from django.db import connection
+from django.test import Client
+from django_scopes import scopes_disabled
+
+from eventyay.base.models.orders import OrderPayment
+
+
+@pytest.mark.django_db(transaction=True)
+class TestConcurrentRefunds:
+    """Test suite for concurrent refund race condition prevention."""
+
+    def test_concurrent_refunds_race_condition_prevented(
+        self, token_client, organizer, event, order
+    ):
+        """
+        Test that concurrent refund requests are properly serialized.
+        
+        Two threads attempt to refund $80 each from a $100 payment.
+        With select_for_update(): only one succeeds, preventing double refund.
+        """
+        # Set up a payment with confirmed state
+        with scopes_disabled():
+            payment = order.payments.first()
+            payment.amount = Decimal('100.00')
+            payment.state = OrderPayment.PAYMENT_STATE_CONFIRMED
+            payment.save()
+
+        results = []
+        errors = []
+        auth_header = token_client.defaults.get('HTTP_AUTHORIZATION')
+        start_barrier = threading.Barrier(2)
+
+        def make_refund_request(amount, barrier):
+            """Make a refund request in a separate thread."""
+            try:
+                barrier.wait()
+                
+                client = Client()
+                client.defaults['HTTP_AUTHORIZATION'] = auth_header
+                response = client.post(
+                    f'/api/v1/organizers/{organizer.slug}/events/{event.slug}/'
+                    f'orders/{order.code}/payments/{payment.local_id}/refund/',
+                    data=json.dumps({'amount': str(amount)}),
+                    content_type='application/json',
+                )
+                results.append({
+                    'status': response.status_code,
+                    'amount': amount,
+                    'body': response.content.decode('utf-8')[:500]
+                })
+            except Exception as e:
+                errors.append(str(e))
+            finally:
+                connection.close()
+
+        thread1 = threading.Thread(target=make_refund_request, args=(Decimal('80.00'), start_barrier))
+        thread2 = threading.Thread(target=make_refund_request, args=(Decimal('80.00'), start_barrier))
+
+        thread1.start()
+        thread2.start()
+
+        thread1.join(timeout=10)
+        thread2.join(timeout=10)
+
+        if thread1.is_alive() or thread2.is_alive():
+            pytest.fail(
+                f"Test threads did not finish within timeout. "
+                f"{{thread1 alive: {{thread1.is_alive()}}}}, thread2 alive: {{thread2.is_alive()}}}}"
+            )
+
+        assert len(errors) == 0, f"Thread errors occurred: {errors}"
+        assert len(results) == 2, f"Expected 2 results, got {len(results)}"
+
+        status_codes = sorted([r['status'] for r in results])
+        assert status_codes == [200, 400], (
+            f"Expected [200, 400], got {status_codes}. Results: {results}"
+        )
+
+        with scopes_disabled():
+            payment.refresh_from_db()
+            refunds = list(payment.refunds.all())
+            total_refunded = sum(r.amount for r in refunds)
+
+        assert len(refunds) == 1, (
+            f"Expected 1 refund, got {len(refunds)}. "
+            "Race condition not prevented - select_for_update() may not be working."
+        )
+
+        assert total_refunded == Decimal('80.00'), (
+            f"Expected $80.00 refunded, got ${total_refunded}. Double refund occurred!"
+        )