diff --git a/packages/mui-system/src/Box/Box.d.ts b/packages/mui-system/src/Box/Box.d.ts
index 8fc7bd47d3cc6f..7f65f840a1b9ce 100644
--- a/packages/mui-system/src/Box/Box.d.ts
+++ b/packages/mui-system/src/Box/Box.d.ts
@@ -168,7 +168,7 @@ export type SystemProps<Theme extends object = {}> = {
   [K in StandardSystemKeys]?:
     | ResponsiveStyleValue<AllSystemCSSProperties[K]>
     | ((theme: Theme) => ResponsiveStyleValue<AllSystemCSSProperties[K]>);
-};
+} & CustomSystemProps;
 
 export interface BoxTypeMap<P = {}, D extends React.ElementType = 'div'> {
   props: P &
diff --git a/packages/mui-system/src/Box/Box.spec.tsx b/packages/mui-system/src/Box/Box.spec.tsx
index 4e81f4bd530ea4..644633d3b7085b 100644
--- a/packages/mui-system/src/Box/Box.spec.tsx
+++ b/packages/mui-system/src/Box/Box.spec.tsx
@@ -10,6 +10,16 @@ function Test(props: TestProps) {
   return <span {...other}>{test}</span>;
 }
 
+declare module './Box' {
+  interface CustomSystemProps {
+    customProp?: 'customValue';
+  }
+}
+
+function CustomSystemPropsTest() {
+  <Box customProp="customValue" />;
+}
+
 function ResponsiveTest() {
   <Box sx={{ p: [2, 3, 4] }} />;
   <Box sx={{ p: { xs: 2, sm: 3, md: 4 } }} />;