diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -2781,7 +2781,6 @@
 void
 PyBytes_Concat(PyObject **pv, PyObject *w)
 {
-    PyObject *v;
     assert(pv != NULL);
     if (*pv == NULL)
         return;
@@ -2789,9 +2788,32 @@
         Py_CLEAR(*pv);
         return;
     }
-    v = bytes_concat(*pv, w);
-    Py_DECREF(*pv);
-    *pv = v;
+    if (!PyBytes_Check(*pv) || !PyBytes_Check(w)) {
+        PyErr_BadInternalCall();
+        Py_CLEAR(*pv);
+        return;
+    }
+    
+    if (Py_REFCNT(*pv) == 1) {
+        /* Only one reference, so we can resize in place */
+        PyBytesObject *sv;
+        size_t oldsize, w_size;
+
+        oldsize = PyBytes_GET_SIZE(*pv);
+        w_size = PyBytes_GET_SIZE(w);
+        if (_PyBytes_Resize(pv, oldsize + w_size) != 0)
+            return;
+
+        sv = (PyBytesObject*) *pv;
+        memcpy(sv->ob_sval + oldsize,  PyBytes_AS_STRING(w),
+               w_size);
+    }
+    else {
+        PyObject *v;
+        v = bytes_concat(*pv, w);
+        Py_DECREF(*pv);
+        *pv = v;
+    }
 }
 
 void