diff --git a/Lib/test/test_class.py b/Lib/test/test_class.py
index 4d554a3..ecc01f2 100644
--- a/Lib/test/test_class.py
+++ b/Lib/test/test_class.py
@@ -568,5 +568,32 @@
         a = A(hash(A.f)^(-1))
         hash(a.f)
 
+    def testSetattrWrapperNameIntern(self):
+        # Issue #25794: __setattr__ should intern the attribute name
+        class A:
+            pass
+
+        def add(self, other):
+            return 'summa'
+
+        name = str(b'__add__', 'ascii')  # shouldn't be optimized
+        self.assertIsNot(name, '__add__')  # not interned
+        type.__setattr__(A, name, add)
+        self.assertEqual(A() + 1, 'summa')
+
+        name2 = str(b'__add__', 'ascii')
+        self.assertIsNot(name2, '__add__')
+        self.assertIsNot(name2, name)
+        type.__delattr__(A, name2)
+        with self.assertRaises(TypeError):
+            A() + 1
+
+    def testSetattrNonStringName(self):
+        class A:
+            pass
+
+        with self.assertRaises(TypeError):
+            type.__setattr__(A, b'x', None)
+
 if __name__ == '__main__':
     unittest.main()
diff --git a/Misc/NEWS b/Misc/NEWS
index 6336e67..e1ea9df 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,9 @@
 Core and Builtins
 -----------------
 
+- bpo-25794: Fixed type.__setattr__() and type.__delattr__() for
+  non-interned attribute names.  Based on patch by Eryk Sun.
+
 - bpo-12414: sys.getsizeof() on a code object now returns the sizes
   which includes the code struct and sizes of objects which it references.
   Patch by Dong-hee Na.
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 329261b..271b935 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -3075,6 +3075,7 @@
 static int
 type_setattro(PyTypeObject *type, PyObject *name, PyObject *value)
 {
+    int res;
     if (!(type->tp_flags & Py_TPFLAGS_HEAPTYPE)) {
         PyErr_Format(
             PyExc_TypeError,
@@ -3082,9 +3083,35 @@
             type->tp_name);
         return -1;
     }
-    if (PyObject_GenericSetAttr((PyObject *)type, name, value) < 0)
-        return -1;
-    return update_slot(type, name);
+    if (PyUnicode_Check(name)) {
+        if (PyUnicode_CheckExact(name)) {
+            if (PyUnicode_READY(name) == -1)
+                return -1;
+            Py_INCREF(name);
+        }
+        else {
+            name = _PyUnicode_Copy(name);
+            if (name == NULL)
+                return -1;
+        }
+        PyUnicode_InternInPlace(&name);
+        if (!PyUnicode_CHECK_INTERNED(name)) {
+            PyErr_SetString(PyExc_MemoryError,
+                            "Out of memory interning an attribute name");
+            Py_DECREF(name);
+            return -1;
+        }
+    }
+    else {
+        /* Will fail in _PyObject_GenericSetAttrWithDict. */
+        Py_INCREF(name);
+    }
+    res = PyObject_GenericSetAttr((PyObject *)type, name, value);
+    if (res == 0) {
+        res = update_slot(type, name);
+    }
+    Py_DECREF(name);
+    return res;
 }
 
 extern void
@@ -6929,7 +6956,7 @@
         /* Slots must be ordered by their offset in the PyHeapTypeObject. */
         assert(!p[1].name || p->offset <= p[1].offset);
         p->name_strobj = PyUnicode_InternFromString(p->name);
-        if (!p->name_strobj)
+        if (!p->name_strobj || !PyUnicode_CHECK_INTERNED(p->name_strobj))
             Py_FatalError("Out of memory interning slotdef names");
     }
     slotdefs_initialized = 1;
@@ -6954,6 +6981,9 @@
     slotdef **pp;
     int offset;
 
+    assert(PyUnicode_CheckExact(name));
+    assert(PyUnicode_CHECK_INTERNED(name));
+
     /* Clear the VALID_VERSION flag of 'type' and all its
        subclasses.  This could possibly be unified with the
        update_subclasses() recursion below, but carefully:
@@ -6964,7 +6994,6 @@
     init_slotdefs();
     pp = ptrs;
     for (p = slotdefs; p->name; p++) {
-        /* XXX assume name is interned! */
         if (p->name_strobj == name)
             *pp++ = p;
     }