diff --git a/packages/mui-system/src/merge/merge.d.ts b/packages/mui-system/src/merge/merge.d.ts
deleted file mode 100644
index f9df3fd9f021f5..00000000000000
--- a/packages/mui-system/src/merge/merge.d.ts
+++ /dev/null
@@ -1 +0,0 @@
-export default function merge(acc: object, item: object): object;
diff --git a/packages/mui-system/src/merge/merge.js b/packages/mui-system/src/merge/merge.ts
similarity index 79%
rename from packages/mui-system/src/merge/merge.js
rename to packages/mui-system/src/merge/merge.ts
index 8fdd03b1d17c18..ed91918dd1657c 100644
--- a/packages/mui-system/src/merge/merge.js
+++ b/packages/mui-system/src/merge/merge.ts
@@ -1,6 +1,6 @@
 import deepmerge from '@mui/utils/deepmerge';
 
-function merge(acc, item) {
+function merge(acc: object, item: object): object {
   if (!item) {
     return acc;
   }
diff --git a/packages/mui-system/src/responsivePropType/responsivePropType.d.ts b/packages/mui-system/src/responsivePropType/responsivePropType.d.ts
deleted file mode 100644
index e6d62effc86140..00000000000000
--- a/packages/mui-system/src/responsivePropType/responsivePropType.d.ts
+++ /dev/null
@@ -1,3 +0,0 @@
-declare const responsivePropType: object;
-
-export default responsivePropType;
diff --git a/packages/mui-system/src/responsivePropType/responsivePropType.js b/packages/mui-system/src/responsivePropType/responsivePropType.ts
similarity index 86%
rename from packages/mui-system/src/responsivePropType/responsivePropType.js
rename to packages/mui-system/src/responsivePropType/responsivePropType.ts
index 5a381b0463267e..e59380f7a9dc47 100644
--- a/packages/mui-system/src/responsivePropType/responsivePropType.js
+++ b/packages/mui-system/src/responsivePropType/responsivePropType.ts
@@ -1,6 +1,6 @@
 import PropTypes from 'prop-types';
 
-const responsivePropType =
+const responsivePropType: object =
   process.env.NODE_ENV !== 'production'
     ? PropTypes.oneOfType([PropTypes.number, PropTypes.string, PropTypes.object, PropTypes.array])
     : {};