diff --git a/Doc/c-api/unicode.rst b/Doc/c-api/unicode.rst
index 4845e0f300278d..3df9d14552e74a 100644
--- a/Doc/c-api/unicode.rst
+++ b/Doc/c-api/unicode.rst
@@ -1867,7 +1867,7 @@ object.
    On success, return ``0``.
    On error, set an exception, leave the writer unchanged, and return ``-1``.
 
-.. c:function:: int PyUnicodeWriter_WriteUCS4(PyUnicodeWriter *writer, Py_UCS4 *str, Py_ssize_t size)
+.. c:function:: int PyUnicodeWriter_WriteUCS4(PyUnicodeWriter *writer, const Py_UCS4 *str, Py_ssize_t size)
 
    Writer the UCS4 string *str* into *writer*.
 
diff --git a/Include/cpython/unicodeobject.h b/Include/cpython/unicodeobject.h
index 631a6570658410..ea91f4158eb392 100644
--- a/Include/cpython/unicodeobject.h
+++ b/Include/cpython/unicodeobject.h
@@ -496,7 +496,7 @@ PyAPI_FUNC(int) PyUnicodeWriter_WriteWideChar(
     Py_ssize_t size);
 PyAPI_FUNC(int) PyUnicodeWriter_WriteUCS4(
     PyUnicodeWriter *writer,
-    Py_UCS4 *str,
+    const Py_UCS4 *str,
     Py_ssize_t size);
 
 PyAPI_FUNC(int) PyUnicodeWriter_WriteStr(
diff --git a/Misc/NEWS.d/next/C_API/2026-03-18-23-44-29.gh-issue-146143.pwIrJq.rst b/Misc/NEWS.d/next/C_API/2026-03-18-23-44-29.gh-issue-146143.pwIrJq.rst
new file mode 100644
index 00000000000000..930d90ff9a2675
--- /dev/null
+++ b/Misc/NEWS.d/next/C_API/2026-03-18-23-44-29.gh-issue-146143.pwIrJq.rst
@@ -0,0 +1,2 @@
+:c:func:`PyUnicodeWriter_WriteUCS4` now accepts a pointer to a constant buffer
+of ``Py_UCS4``.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index dd2482ca653fb1..d51a95c69a93b3 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -2224,7 +2224,7 @@ _PyUnicode_FromUCS4(const Py_UCS4 *u, Py_ssize_t size)
 
 int
 PyUnicodeWriter_WriteUCS4(PyUnicodeWriter *pub_writer,
-                          Py_UCS4 *str,
+                          const Py_UCS4 *str,
                           Py_ssize_t size)
 {
     _PyUnicodeWriter *writer = (_PyUnicodeWriter*)pub_writer;