diff --git a/Lib/base64.py b/Lib/base64.py
index 5d78cc09f40cd3..cfc57626c40ba9 100644
--- a/Lib/base64.py
+++ b/Lib/base64.py
@@ -462,9 +462,12 @@ def b85decode(b):
     # Delay the initialization of tables to not waste memory
     # if the function is never called
     if _b85dec is None:
-        _b85dec = [None] * 256
+        # we don't assign to _b85dec directly to avoid issues when
+        # multiple threads call this function simultaneously
+        b85dec_tmp = [None] * 256
         for i, c in enumerate(_b85alphabet):
-            _b85dec[c] = i
+            b85dec_tmp[c] = i
+        _b85dec = b85dec_tmp
 
     b = _bytes_from_decode_data(b)
     padding = (-len(b)) % 5
diff --git a/Misc/NEWS.d/next/Library/2025-11-06-15-11-50.gh-issue-141141.tgIfgH.rst b/Misc/NEWS.d/next/Library/2025-11-06-15-11-50.gh-issue-141141.tgIfgH.rst
new file mode 100644
index 00000000000000..f59ccfb33e7669
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-11-06-15-11-50.gh-issue-141141.tgIfgH.rst
@@ -0,0 +1 @@
+Fix a thread safety issue with :func:`base64.b85decode`. Contributed by Benel Tayar.