diff --git a/Lib/csv.py b/Lib/csv.py
index a56eed88c86..c3c31f01fd0 100644
--- a/Lib/csv.py
+++ b/Lib/csv.py
@@ -231,20 +231,21 @@ class Sniffer:
         quotes = {}
         delims = {}
         spaces = 0
+        groupindex = regexp.groupindex
         for m in matches:
-            n = regexp.groupindex['quote'] - 1
+            n = groupindex['quote'] - 1
             key = m[n]
             if key:
                 quotes[key] = quotes.get(key, 0) + 1
             try:
-                n = regexp.groupindex['delim'] - 1
+                n = groupindex['delim'] - 1
                 key = m[n]
             except KeyError:
                 continue
             if key and (delimiters is None or key in delimiters):
                 delims[key] = delims.get(key, 0) + 1
             try:
-                n = regexp.groupindex['space'] - 1
+                n = groupindex['space'] - 1
             except KeyError:
                 continue
             if m[n]:
diff --git a/Lib/sre_parse.py b/Lib/sre_parse.py
index 0bce71e9c97..c0f539dd931 100644
--- a/Lib/sre_parse.py
+++ b/Lib/sre_parse.py
@@ -855,6 +855,7 @@ def parse_template(source, pattern):
             del literal[:]
         groups.append((len(literals), index))
         literals.append(None)
+    groupindex = pattern.groupindex
     while True:
         this = sget()
         if this is None:
@@ -869,7 +870,7 @@ def parse_template(source, pattern):
                 name = s.getuntil(">")
                 if name.isidentifier():
                     try:
-                        index = pattern.groupindex[name]
+                        index = groupindex[name]
                     except KeyError:
                         raise IndexError("unknown group name %r" % name)
                 else:
diff --git a/Lib/test/test_re.py b/Lib/test/test_re.py
index 020656a62a4..5b716125f97 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -577,6 +577,14 @@ class ReTests(unittest.TestCase):
         self.assertEqual(re.match("(a)", "a").regs, ((0, 1), (0, 1)))
         self.assertTrue(re.match("(a)", "a").re)
 
+        # Issue 14260. groupindex should be non-modifiable mapping.
+        p = re.compile(r'(?i)(?P<first>a)(?P<other>b)')
+        self.assertEqual(sorted(p.groupindex), ['first', 'other'])
+        self.assertEqual(p.groupindex['other'], 2)
+        with self.assertRaises(TypeError):
+            p.groupindex['other'] = 0
+        self.assertEqual(p.groupindex['other'], 2)
+
     def test_special_escapes(self):
         self.assertEqual(re.search(r"\b(b.)\b",
                                    "abcd abc bcd bx").group(1), "bx")
diff --git a/Misc/NEWS b/Misc/NEWS
index 333f6457ee3..351a07fb808 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -30,6 +30,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #14260: The groupindex attribute of regular expression pattern object
+  now is non-modifiable mapping.
+
 - Issue #23792: Ignore KeyboardInterrupt when the pydoc pager is active.
   This mimics the behavior of the standard unix pagers, and prevents
   pipepager from shutting down while the pager itself is still running.
diff --git a/Modules/_sre.c b/Modules/_sre.c
index 83eb96376a7..8cad1ac270b 100644
--- a/Modules/_sre.c
+++ b/Modules/_sre.c
@@ -1384,12 +1384,24 @@ static PyMethodDef pattern_methods[] = {
     {NULL, NULL}
 };
 
+/* PatternObject's 'groupindex' method. */
+static PyObject *
+pattern_groupindex(PatternObject *self)
+{
+    return PyDictProxy_New(self->groupindex);
+}
+
+static PyGetSetDef pattern_getset[] = {
+    {"groupindex", (getter)pattern_groupindex, (setter)NULL,
+      "A dictionary mapping group names to group numbers."},
+    {NULL}  /* Sentinel */
+};
+
 #define PAT_OFF(x) offsetof(PatternObject, x)
 static PyMemberDef pattern_members[] = {
     {"pattern",    T_OBJECT,    PAT_OFF(pattern),       READONLY},
     {"flags",      T_INT,       PAT_OFF(flags),         READONLY},
     {"groups",     T_PYSSIZET,  PAT_OFF(groups),        READONLY},
-    {"groupindex", T_OBJECT,    PAT_OFF(groupindex),    READONLY},
     {NULL}  /* Sentinel */
 };
 
@@ -1422,6 +1434,7 @@ static PyTypeObject Pattern_Type = {
     0,                                  /* tp_iternext */
     pattern_methods,                    /* tp_methods */
     pattern_members,                    /* tp_members */
+    pattern_getset,                     /* tp_getset */
 };
 
 static int _validate(PatternObject *self); /* Forward */