diff --git a/Doc/c-api/arg.rst b/Doc/c-api/arg.rst
index 81b093a3510914..d68dc0885681df 100644
--- a/Doc/c-api/arg.rst
+++ b/Doc/c-api/arg.rst
@@ -669,6 +669,8 @@ Building values
    ``L`` (:class:`int`) [long long]
       Convert a C :c:expr:`long long` to a Python integer object.
 
+   .. _capi-py-buildvalue-format-K:
+
    ``K`` (:class:`int`) [unsigned long long]
       Convert a C :c:expr:`unsigned long long` to a Python integer object.
 
diff --git a/Misc/NEWS.d/next/C_API/2025-04-25-11-39-24.gh-issue-132909.JC3n_l.rst b/Misc/NEWS.d/next/C_API/2025-04-25-11-39-24.gh-issue-132909.JC3n_l.rst
new file mode 100644
index 00000000000000..81a37d0595e2e0
--- /dev/null
+++ b/Misc/NEWS.d/next/C_API/2025-04-25-11-39-24.gh-issue-132909.JC3n_l.rst
@@ -0,0 +1,2 @@
+Fix an overflow when handling the :ref:`K <capi-py-buildvalue-format-K>` format
+in :c:func:`Py_BuildValue`. Patch by Bénédikt Tran.
diff --git a/Python/modsupport.c b/Python/modsupport.c
index 501231affe8cd4..2caf595949d52f 100644
--- a/Python/modsupport.c
+++ b/Python/modsupport.c
@@ -321,7 +321,8 @@ do_mkvalue(const char **p_format, va_list *p_va)
             return PyLong_FromLongLong((long long)va_arg(*p_va, long long));
 
         case 'K':
-            return PyLong_FromUnsignedLongLong((long long)va_arg(*p_va, unsigned long long));
+            return PyLong_FromUnsignedLongLong(
+                va_arg(*p_va, unsigned long long));
 
         case 'u':
         {