diff --git a/Misc/NEWS.d/next/Library/2024-06-04-19-03-25.gh-issue-112672.K2XfZH.rst b/Misc/NEWS.d/next/Library/2024-06-04-19-03-25.gh-issue-112672.K2XfZH.rst
new file mode 100644
index 00000000000..46345bff117
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-06-04-19-03-25.gh-issue-112672.K2XfZH.rst
@@ -0,0 +1 @@
+Support building :mod:`tkinter` with Tcl 9.0.
diff --git a/Modules/_tkinter.c b/Modules/_tkinter.c
index 0cff36dd307..24f87c8d34c 100644
--- a/Modules/_tkinter.c
+++ b/Modules/_tkinter.c
@@ -69,6 +69,12 @@ Copyright (C) 1994 Steen Lumholt.
 #define USE_DEPRECATED_TOMMATH_API 1
 #endif
 
+// As suggested by https://core.tcl-lang.org/tcl/wiki?name=Migrating+C+extensions+to+Tcl+9
+#ifndef TCL_SIZE_MAX
+typedef int Tcl_Size;
+#define TCL_SIZE_MAX INT_MAX
+#endif
+
 #if !(defined(MS_WINDOWS) || defined(__CYGWIN__))
 #define HAVE_CREATEFILEHANDLER
 #endif
@@ -489,7 +495,7 @@ unicodeFromTclString(const char *s)
 static PyObject *
 unicodeFromTclObj(Tcl_Obj *value)
 {
-    int len;
+    Tcl_Size len;
 #if USE_TCL_UNICODE
     int byteorder = NATIVE_BYTEORDER;
     const Tcl_UniChar *u = Tcl_GetUnicodeFromObj(value, &len);
@@ -517,6 +523,10 @@ class _tkinter.tktimertoken "TkttObject *" "&Tktt_Type_spec"
 
 /**** Tkapp Object ****/
 
+#if TK_MAJOR_VERSION >= 9
+int Tcl_AppInit(Tcl_Interp *);
+#endif
+
 #ifndef WITH_APPINIT
 int
 Tcl_AppInit(Tcl_Interp *interp)
@@ -1142,7 +1152,7 @@ FromObj(TkappObject *tkapp, Tcl_Obj *value)
     }
 
     if (value->typePtr == tkapp->ByteArrayType) {
-        int size;
+        Tcl_Size size;
         char *data = (char*)Tcl_GetByteArrayFromObj(value, &size);
         return PyBytes_FromStringAndSize(data, size);
     }
@@ -1168,8 +1178,8 @@ FromObj(TkappObject *tkapp, Tcl_Obj *value)
     }
 
     if (value->typePtr == tkapp->ListType) {
-        int size;
-        int i, status;
+        Tcl_Size i, size;
+        int status;
         PyObject *elem;
         Tcl_Obj *tcl_elem;
 
@@ -1225,9 +1235,9 @@ typedef struct Tkapp_CallEvent {
 } Tkapp_CallEvent;
 
 static void
-Tkapp_CallDeallocArgs(Tcl_Obj** objv, Tcl_Obj** objStore, int objc)
+Tkapp_CallDeallocArgs(Tcl_Obj** objv, Tcl_Obj** objStore, Tcl_Size objc)
 {
-    int i;
+    Tcl_Size i;
     for (i = 0; i < objc; i++)
         Tcl_DecrRefCount(objv[i]);
     if (objv != objStore)
@@ -1238,7 +1248,7 @@ Tkapp_CallDeallocArgs(Tcl_Obj** objv, Tcl_Obj** objStore, int objc)
    interpreter thread, which may or may not be the calling thread. */
 
 static Tcl_Obj**
-Tkapp_CallArgs(PyObject *args, Tcl_Obj** objStore, int *pobjc)
+Tkapp_CallArgs(PyObject *args, Tcl_Obj** objStore, Tcl_Size *pobjc)
 {
     Tcl_Obj **objv = objStore;
     Py_ssize_t objc = 0, i;
@@ -1286,10 +1296,10 @@ Tkapp_CallArgs(PyObject *args, Tcl_Obj** objStore, int *pobjc)
             Tcl_IncrRefCount(objv[i]);
         }
     }
-    *pobjc = (int)objc;
+    *pobjc = (Tcl_Size)objc;
     return objv;
 finally:
-    Tkapp_CallDeallocArgs(objv, objStore, (int)objc);
+    Tkapp_CallDeallocArgs(objv, objStore, (Tcl_Size)objc);
     return NULL;
 }
 
@@ -1356,7 +1366,7 @@ Tkapp_CallProc(Tcl_Event *evPtr, int flags)
     Tkapp_CallEvent *e = (Tkapp_CallEvent *)evPtr;
     Tcl_Obj *objStore[ARGSZ];
     Tcl_Obj **objv;
-    int objc;
+    Tcl_Size objc;
     int i;
     ENTER_PYTHON
     if (e->self->trace && !Tkapp_Trace(e->self, PyTuple_Pack(1, e->args))) {
@@ -1412,7 +1422,7 @@ Tkapp_Call(PyObject *selfptr, PyObject *args)
 {
     Tcl_Obj *objStore[ARGSZ];
     Tcl_Obj **objv = NULL;
-    int objc, i;
+    Tcl_Size objc;
     PyObject *res = NULL;
     TkappObject *self = (TkappObject*)selfptr;
     int flags = TCL_EVAL_DIRECT | TCL_EVAL_GLOBAL;
@@ -1459,6 +1469,7 @@ Tkapp_Call(PyObject *selfptr, PyObject *args)
     {
         TRACE(self, ("(O)", args));
 
+        int i;
         objv = Tkapp_CallArgs(args, objStore, &objc);
         if (!objv)
             return NULL;
@@ -2193,13 +2204,12 @@ _tkinter_tkapp_splitlist(TkappObject *self, PyObject *arg)
 /*[clinic end generated code: output=13b51d34386d36fb input=2b2e13351e3c0b53]*/
 {
     char *list;
-    int argc;
+    Tcl_Size argc, i;
     const char **argv;
     PyObject *v;
-    int i;
 
     if (PyTclObject_Check(arg)) {
-        int objc;
+        Tcl_Size objc;
         Tcl_Obj **objv;
         if (Tcl_ListObjGetElements(Tkapp_Interp(self),
                                    ((PyTclObject*)arg)->value,