diff --git a/src/net/http/transport.go b/src/net/http/transport.go
index a90f36ff73..b8e4c4e97b 100644
--- a/src/net/http/transport.go
+++ b/src/net/http/transport.go
@@ -622,6 +622,12 @@ func (t *Transport) roundTrip(req *Request) (*Response, error) {
 			if e, ok := err.(transportReadFromServerError); ok {
 				err = e.err
 			}
+			if b, ok := req.Body.(*readTrackingBody); ok && !b.didClose {
+				// Issue 49621: Close the request body if pconn.roundTrip
+				// didn't do so already. This can happen if the pconn
+				// write loop exits without reading the write request.
+				req.closeBody()
+			}
 			return nil, err
 		}
 		testHookRoundTripRetried()
diff --git a/src/net/http/transport_test.go b/src/net/http/transport_test.go
index 245f73bc9f..2879dee0fd 100644
--- a/src/net/http/transport_test.go
+++ b/src/net/http/transport_test.go
@@ -4092,6 +4092,45 @@ func testTransportDialCancelRace(t *testing.T, mode testMode) {
 	}
 }
 
+// https://go.dev/issue/49621
+func TestConnClosedBeforeRequestIsWritten(t *testing.T) {
+	run(t, testConnClosedBeforeRequestIsWritten, testNotParallel, []testMode{http1Mode})
+}
+func testConnClosedBeforeRequestIsWritten(t *testing.T, mode testMode) {
+	ts := newClientServerTest(t, mode, HandlerFunc(func(w ResponseWriter, r *Request) {}),
+		func(tr *Transport) {
+			tr.DialContext = func(_ context.Context, network, addr string) (net.Conn, error) {
+				// Connection immediately returns errors.
+				return &funcConn{
+					read: func([]byte) (int, error) {
+						return 0, errors.New("error")
+					},
+					write: func([]byte) (int, error) {
+						return 0, errors.New("error")
+					},
+				}, nil
+			}
+		},
+	).ts
+	// Set a short delay in RoundTrip to give the persistConn time to notice
+	// the connection is broken. We want to exercise the path where writeLoop exits
+	// before it reads the request to send. If this delay is too short, we may instead
+	// exercise the path where writeLoop accepts the request and then fails to write it.
+	// That's fine, so long as we get the desired path often enough.
+	SetEnterRoundTripHook(func() {
+		time.Sleep(1 * time.Millisecond)
+	})
+	defer SetEnterRoundTripHook(nil)
+	var closes int
+	_, err := ts.Client().Post(ts.URL, "text/plain", countCloseReader{&closes, strings.NewReader("hello")})
+	if err == nil {
+		t.Fatalf("expected request to fail, but it did not")
+	}
+	if closes != 1 {
+		t.Errorf("after RoundTrip, request body was closed %v times; want 1", closes)
+	}
+}
+
 // logWritesConn is a net.Conn that logs each Write call to writes
 // and then proxies to w.
 // It proxies Read calls to a reader it receives from rch.