diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6acbd3b103..b860dc239d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@
 
 - Add `lastEventId` method to the API ([#2675](https://github.com/getsentry/sentry-react-native/pull/2675))
 
+### Fix
+
+- `Sentry.startTransaction` doesn't require `op` ([#2691](https://github.com/getsentry/sentry-react-native/pull/2691))
+
 ### Dependencies
 
 - Bump Cocoa SDK from v7.31.2 to v7.31.3 ([#2647](https://github.com/getsentry/sentry-react-native/pull/2647))
diff --git a/src/js/measurements.ts b/src/js/measurements.ts
index 242d7b3add..709a9122a0 100644
--- a/src/js/measurements.ts
+++ b/src/js/measurements.ts
@@ -1,9 +1,11 @@
 import { getCurrentHub, getMainCarrier, Hub } from '@sentry/core';
 import { Transaction } from '@sentry/tracing';
-import { CustomSamplingContext, TransactionContext } from '@sentry/types';
+import { CustomSamplingContext, Span, SpanContext, TransactionContext } from '@sentry/types';
 
 import { ReactNativeTracing } from './tracing';
 
+const SPAN_OP_DEFAULT = 'default';
+
 /**
  * Adds React Native's extensions. Needs to be called after @sentry/tracing's extension methods are added
  */
@@ -28,7 +30,7 @@ export function _addTracingExtensions(): void {
   }
 }
 
-type StartTransactionFunction = (
+export type StartTransactionFunction = (
   this: Hub,
   transactionContext: TransactionContext,
   customSamplingContext?: CustomSamplingContext
@@ -48,10 +50,25 @@ const _patchStartTransaction = (
     transactionContext: TransactionContext,
     customSamplingContext?: CustomSamplingContext
   ): Transaction {
+    // Native SDKs require op to be set - for JS Relay sets `default`
+    if (!transactionContext.op) {
+      transactionContext.op = SPAN_OP_DEFAULT;
+    }
+
     const transaction: Transaction = originalStartTransaction.apply(this, [
       transactionContext,
       customSamplingContext,
     ]);
+    const originalStartChild: Transaction['startChild'] = transaction.startChild.bind(transaction);
+    transaction.startChild = (
+      spanContext?: Pick<SpanContext, Exclude<keyof SpanContext, 'sampled' | 'traceId' | 'parentSpanId'>>,
+    ): Span => {
+      return originalStartChild({
+        ...spanContext,
+        // Native SDKs require op to be set
+        op: spanContext?.op || SPAN_OP_DEFAULT,
+      });
+    };
 
     const reactNativeTracing = getCurrentHub().getIntegration(
       ReactNativeTracing
diff --git a/test/measurements.test.ts b/test/measurements.test.ts
new file mode 100644
index 0000000000..cbe6ac8d5a
--- /dev/null
+++ b/test/measurements.test.ts
@@ -0,0 +1,56 @@
+import { Carrier, getCurrentHub, getMainCarrier } from '@sentry/core';
+import { Transaction } from '@sentry/tracing';
+import { Hub } from '@sentry/types';
+
+import { _addTracingExtensions, StartTransactionFunction } from '../src/js/measurements';
+
+describe('Tracing extensions', () => {
+  let hub: Hub;
+  let carrier: Carrier;
+  let startTransaction: StartTransactionFunction | undefined;
+
+  beforeEach(() => {
+    _addTracingExtensions();
+    hub = getCurrentHub();
+    carrier = getMainCarrier();
+    startTransaction = carrier.__SENTRY__?.extensions?.startTransaction as StartTransactionFunction | undefined;
+  });
+
+  test('transaction has default op', async () => {
+    const transaction: Transaction = startTransaction?.apply(hub, [{}]);
+
+    expect(transaction).toEqual(expect.objectContaining({ op: 'default' }));
+  });
+
+  test('transaction does not overwrite custom op', async () => {
+    const transaction: Transaction = startTransaction?.apply(hub, [{ op: 'custom' }]);
+
+    expect(transaction).toEqual(expect.objectContaining({ op: 'custom' }));
+  });
+
+  test('transaction start span creates default op', async () => {
+    const transaction: Transaction = startTransaction?.apply(hub, [{ op: 'custom' }]);
+    const span = transaction?.startChild();
+
+    expect(span).toEqual(expect.objectContaining({ op: 'default' }));
+  });
+
+  test('transaction start span keeps custom op', async () => {
+    const transaction: Transaction = startTransaction?.apply(hub, [{ op: 'custom' }]);
+    const span = transaction?.startChild({ op: 'custom' });
+
+    expect(span).toEqual(expect.objectContaining({ op: 'custom' }));
+  });
+
+  test('transaction start span passes correct values to the child', async () => {
+    const transaction: Transaction = startTransaction?.apply(hub, [{ op: 'custom' }]);
+    const span = transaction?.startChild({ op: 'custom' });
+
+    expect(span).toEqual(expect.objectContaining({
+      transaction,
+      parentSpanId: transaction.spanId,
+      sampled: transaction.sampled,
+      traceId: transaction.traceId,
+    }));
+  });
+});