diff --git a/Misc/NEWS.d/next/Library/2023-05-11-23-03-00.gh-issue-104399.MMatTP.rst b/Misc/NEWS.d/next/Library/2023-05-11-23-03-00.gh-issue-104399.MMatTP.rst
new file mode 100644
index 00000000000..84cc888635b
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-05-11-23-03-00.gh-issue-104399.MMatTP.rst
@@ -0,0 +1,4 @@
+Prepare the ``_tkinter`` module for building with Tcl 9.0 and future
+libtommath by replacing usage of deprecated functions
+:c:func:`mp_to_unsigned_bin_n` and :c:func:`mp_unsigned_bin_size`
+when necessary.
diff --git a/Modules/_tkinter.c b/Modules/_tkinter.c
index db57d0d7204..97e5b2f738a 100644
--- a/Modules/_tkinter.c
+++ b/Modules/_tkinter.c
@@ -65,6 +65,12 @@ Copyright (C) 1994 Steen Lumholt.
 #endif
 #include <tclTomMath.h>
 
+#if defined(TCL_WITH_EXTERNAL_TOMMATH) || (TK_HEX_VERSION >= 0x08070000)
+#define USE_DEPRECATED_TOMMATH_API 0
+#else
+#define USE_DEPRECATED_TOMMATH_API 1
+#endif
+
 #if !(defined(MS_WINDOWS) || defined(__CYGWIN__))
 #define HAVE_CREATEFILEHANDLER
 #endif
@@ -1049,20 +1055,33 @@ static PyObject*
 fromBignumObj(TkappObject *tkapp, Tcl_Obj *value)
 {
     mp_int bigValue;
+    mp_err err;
+#if USE_DEPRECATED_TOMMATH_API
     unsigned long numBytes;
+#else
+    size_t numBytes;
+#endif
     unsigned char *bytes;
     PyObject *res;
 
     if (Tcl_GetBignumFromObj(Tkapp_Interp(tkapp), value, &bigValue) != TCL_OK)
         return Tkinter_Error(tkapp);
+#if USE_DEPRECATED_TOMMATH_API
     numBytes = mp_unsigned_bin_size(&bigValue);
+#else
+    numBytes = mp_ubin_size(&bigValue);
+#endif
     bytes = PyMem_Malloc(numBytes);
     if (bytes == NULL) {
         mp_clear(&bigValue);
         return PyErr_NoMemory();
     }
-    if (mp_to_unsigned_bin_n(&bigValue, bytes,
-                                &numBytes) != MP_OKAY) {
+#if USE_DEPRECATED_TOMMATH_API
+    err = mp_to_unsigned_bin_n(&bigValue, bytes, &numBytes);
+#else
+    err = mp_to_ubin(&bigValue, bytes, numBytes, NULL);
+#endif
+    if (err != MP_OKAY) {
         mp_clear(&bigValue);
         PyMem_Free(bytes);
         return PyErr_NoMemory();