diff --git a/dev-packages/e2e-tests/test-applications/nextjs-15/tests/middleware.test.ts b/dev-packages/e2e-tests/test-applications/nextjs-15/tests/middleware.test.ts
index dcfe67d46d5e..533af644c9d1 100644
--- a/dev-packages/e2e-tests/test-applications/nextjs-15/tests/middleware.test.ts
+++ b/dev-packages/e2e-tests/test-applications/nextjs-15/tests/middleware.test.ts
@@ -45,3 +45,36 @@ test('does not leak normalizedRequest between concurrent middleware invocations'
   expect(firstTransaction.request?.url).not.toContain('/api/endpoint-behind-middleware-2');
   expect(secondTransaction.request?.url).toContain('/api/endpoint-behind-middleware-2');
 });
+
+// Neither request sends inbound tracing headers, so each is the head of its own distributed trace. If concurrent
+// middleware invocations were to share an active span/scope (e.g. a leaked context on a warm edge worker), both
+// transactions would inherit one trace id and collapse into a single trace - the production contamination we guard
+// against here.
+test('concurrent middleware invocations without inbound tracing headers get distinct trace ids', async ({
+  request,
+}) => {
+  const firstTransactionPromise = waitForTransaction('nextjs-15', async transactionEvent => {
+    return (
+      transactionEvent?.transaction === 'middleware GET' &&
+      transactionEvent.contexts?.trace?.data?.['http.target'] === '/api/endpoint-behind-middleware'
+    );
+  });
+
+  const secondTransactionPromise = waitForTransaction('nextjs-15', async transactionEvent => {
+    return (
+      transactionEvent?.transaction === 'middleware GET' &&
+      transactionEvent.contexts?.trace?.data?.['http.target'] === '/api/endpoint-behind-middleware-2'
+    );
+  });
+
+  await Promise.all([request.get('/api/endpoint-behind-middleware'), request.get('/api/endpoint-behind-middleware-2')]);
+
+  const [firstTransaction, secondTransaction] = await Promise.all([firstTransactionPromise, secondTransactionPromise]);
+
+  const firstTraceId = firstTransaction.contexts?.trace?.trace_id;
+  const secondTraceId = secondTransaction.contexts?.trace?.trace_id;
+
+  expect(firstTraceId).toMatch(/^[a-f0-9]{32}$/);
+  expect(secondTraceId).toMatch(/^[a-f0-9]{32}$/);
+  expect(firstTraceId).not.toBe(secondTraceId);
+});
diff --git a/packages/vercel-edge/test/middlewareTraceIsolation.test.ts b/packages/vercel-edge/test/middlewareTraceIsolation.test.ts
new file mode 100644
index 000000000000..503c692a7a8d
--- /dev/null
+++ b/packages/vercel-edge/test/middlewareTraceIsolation.test.ts
@@ -0,0 +1,165 @@
+import { context, propagation, ROOT_CONTEXT, trace } from '@opentelemetry/api';
+import { getCurrentScope, getGlobalScope, getIsolationScope, GLOBAL_OBJ, spanToJSON } from '@sentry/core';
+import { setOpenTelemetryContextAsyncContextStrategy } from '@sentry/opentelemetry';
+import { AsyncLocalStorage } from 'async_hooks';
+import { beforeAll, beforeEach, describe, expect, it } from 'vitest';
+import { VercelEdgeClient } from '../src';
+import { setupOtel } from '../src/sdk';
+import { makeEdgeTransport } from '../src/transports';
+
+// These tests reproduce, on the real Sentry OTEL edge stack (SentryTracerProvider + SentryPropagator +
+// AsyncLocalStorage context manager), the exact chain Next.js runs for edge middleware:
+//
+//   adapter -> tracer.withPropagatedContext(request.headers, fn)
+//           -> tracer.trace('Middleware.execute', ..., fn)   // <- this is the root span / transaction
+//
+// The `Middleware.execute` span is the transaction; its trace id is fixed at creation from the parent found on
+// `context.active()`. If concurrent, header-less requests were to ever share that trace id, unrelated requests would
+// collapse into one trace (thousands of transaction names under a single trace id in production).
+
+const headersGetter = {
+  keys: (headers: Headers): string[] => Array.from(headers.keys()),
+  get: (headers: Headers, key: string): string | undefined => headers.get(key) ?? undefined,
+};
+
+// Mirrors next/dist/server/lib/trace/tracer.js `withPropagatedContext`.
+function withPropagatedContext<T>(headers: Headers, fn: () => T): T {
+  const activeContext = context.active();
+  if (trace.getSpanContext(activeContext)) {
+    return fn();
+  }
+  const remoteContext = propagation.extract(activeContext, headers, headersGetter);
+  return context.with(remoteContext, fn);
+}
+
+// Mirrors next/dist/server/lib/trace/tracer.js `trace` for the `Middleware.execute` span.
+function nextMiddlewareTrace<T>(pathname: string, fn: () => T): T {
+  const tracer = trace.getTracer('next.js', '0.0.1');
+  const spanContext = context.active() ?? ROOT_CONTEXT;
+  return context.with(spanContext, () =>
+    tracer.startActiveSpan(
+      `middleware GET ${pathname}`,
+      { attributes: { 'next.span_type': 'Middleware.execute', 'next.span_name': `middleware GET ${pathname}` } },
+      span => {
+        try {
+          return fn();
+        } finally {
+          span.end();
+        }
+      },
+    ),
+  );
+}
+
+// Runs one middleware request end-to-end. `delay` yields to the event loop mid-request so that concurrent requests
+// genuinely interleave (rather than running to completion synchronously one after another).
+async function runMiddlewareRequest(
+  url: string,
+  headers: Record<string, string> = {},
+  delay = 0,
+): Promise<{ traceId: string; childTraceId: string }> {
+  const request = new Request(url, { method: 'GET', headers });
+  return withPropagatedContext(request.headers, () =>
+    nextMiddlewareTrace(new URL(url).pathname, async () => {
+      const rootSpan = trace.getActiveSpan()!;
+      const traceId = spanToJSON(rootSpan).trace_id!;
+
+      await new Promise(resolve => setTimeout(resolve, delay));
+
+      // A child span started within the request must parent onto this request's root span (same trace id), not onto a
+      // concurrent request's span.
+      const childTracer = trace.getTracer('user');
+      const childSpan = childTracer.startSpan('user-work');
+      const childTraceId = spanToJSON(childSpan as unknown as Parameters<typeof spanToJSON>[0]).trace_id!;
+      childSpan.end();
+
+      await new Promise(resolve => setTimeout(resolve, delay));
+
+      return { traceId, childTraceId };
+    }),
+  );
+}
+
+describe('Next.js edge middleware trace isolation', () => {
+  beforeAll(() => {
+    (GLOBAL_OBJ as unknown as { AsyncLocalStorage: typeof AsyncLocalStorage }).AsyncLocalStorage = AsyncLocalStorage;
+
+    const client = new VercelEdgeClient({
+      dsn: 'https://public@dsn.ingest.sentry.io/1337',
+      stackParser: () => [],
+      integrations: [],
+      transport: makeEdgeTransport,
+      tracesSampleRate: 1,
+    });
+
+    setupOtel(client);
+    setOpenTelemetryContextAsyncContextStrategy();
+  });
+
+  beforeEach(() => {
+    getIsolationScope().clear();
+    getCurrentScope().clear();
+    getGlobalScope().clear();
+  });
+
+  it('gives concurrent header-less requests to unrelated routes distinct trace ids', async () => {
+    const urls = [
+      'https://docs.sentry.io/',
+      'https://docs.sentry.io/platforms/python/',
+      'https://docs.sentry.io/product/configuration/',
+      'https://docs.sentry.io/platforms/javascript/',
+    ];
+
+    const results = await Promise.all(urls.map((url, i) => runMiddlewareRequest(url, {}, (i + 1) * 5)));
+    const traceIds = results.map(r => r.traceId);
+
+    expect(traceIds).toHaveLength(urls.length);
+    expect(new Set(traceIds).size).toBe(urls.length);
+    traceIds.forEach(id => expect(id).toMatch(/^[a-f0-9]{32}$/));
+  });
+
+  it('parents a request-local child span onto its own request trace, not a concurrent one', async () => {
+    const results = await Promise.all([
+      runMiddlewareRequest('https://docs.sentry.io/a', {}, 10),
+      runMiddlewareRequest('https://docs.sentry.io/b', {}, 5),
+    ]);
+
+    results.forEach(({ traceId, childTraceId }) => {
+      expect(childTraceId).toBe(traceId);
+    });
+    expect(results[0]!.traceId).not.toBe(results[1]!.traceId);
+  });
+
+  it('continues the incoming trace when each concurrent request carries a distinct sentry-trace header', async () => {
+    const incomingTraceIdA = 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa';
+    const incomingTraceIdB = 'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb';
+
+    const [a, b] = await Promise.all([
+      runMiddlewareRequest(
+        'https://docs.sentry.io/one',
+        { 'sentry-trace': `${incomingTraceIdA}-1111111111111111-1` },
+        5,
+      ),
+      runMiddlewareRequest(
+        'https://docs.sentry.io/two',
+        { 'sentry-trace': `${incomingTraceIdB}-2222222222222222-1` },
+        5,
+      ),
+    ]);
+
+    expect(a!.traceId).toBe(incomingTraceIdA);
+    expect(b!.traceId).toBe(incomingTraceIdB);
+  });
+
+  it('does not join a shared trace when requests carry malformed sentry-trace headers', async () => {
+    const results = await Promise.all([
+      runMiddlewareRequest('https://docs.sentry.io/m1', { 'sentry-trace': 'not-a-valid-trace' }, 5),
+      runMiddlewareRequest('https://docs.sentry.io/m2', { 'sentry-trace': 'garbage-header-value' }, 5),
+    ]);
+    const traceIds = results.map(r => r.traceId);
+
+    expect(traceIds).toHaveLength(2);
+    expect(new Set(traceIds).size).toBe(2);
+    traceIds.forEach(id => expect(id).toMatch(/^[a-f0-9]{32}$/));
+  });
+});