diff --git a/__tests__/Relude_Array_test.re b/__tests__/Relude_Array_test.re
index 4404c2ea..39020676 100644
--- a/__tests__/Relude_Array_test.re
+++ b/__tests__/Relude_Array_test.re
@@ -529,6 +529,11 @@ describe("Array", () => {
     |> toEqual([|6, 1, 2, 3, 4, 5|])
   );
 
+  test("replaceAt", () =>
+    expect(Array.replaceAt(2, 100, [|1, 2, 3, 4, 5|]))
+    |> toEqual([|1, 2, 100, 4, 5|])
+  );
+
   test("map", () =>
     expect(Array.map(a => a + 2, [|1, 2, 3|])) |> toEqual([|3, 4, 5|])
   );
@@ -659,4 +664,4 @@ describe("Array", () => {
          226,
        |])
   );
-});
+});
\ No newline at end of file
diff --git a/__tests__/Relude_List_test.re b/__tests__/Relude_List_test.re
index 56b8d5c5..686394d7 100644
--- a/__tests__/Relude_List_test.re
+++ b/__tests__/Relude_List_test.re
@@ -596,6 +596,11 @@ describe("List", () => {
     |> toEqual([2, 4])
   );
 
+  test("replaceAt", () =>
+    expect(List.replaceAt(2, 100, [1, 2, 3, 4, 5]))
+    |> toEqual([1, 2, 100, 4, 5])
+  );
+
   test("distinctBy", () =>
     expect(List.distinctBy(Int.eq, [6, 1, 1, 2, 1, 3, 2, 3, 2, 4, 5, 5]))
     |> toEqual([6, 1, 2, 3, 4, 5])
diff --git a/src/Relude_Array.rei b/src/Relude_Array.rei
index c7f73efd..c3f3e4ee 100644
--- a/src/Relude_Array.rei
+++ b/src/Relude_Array.rei
@@ -191,6 +191,7 @@ let removeFirst:
 let removeEach:
   ((module BsAbstract.Interface.EQ with type t = 'a), 'a, array('a)) =>
   array('a);
+let replaceAt: (int, 'a, array('a)) => array('a);
 let scanLeft: (('b, 'a) => 'b, 'b, array('a)) => array('b);
 let scanRight: (('a, 'b) => 'b, 'b, array('a)) => array('b);
 module ArrayEqExtensions = Relude_Extensions_Array.ArrayEqExtensions;
diff --git a/src/Relude_List.rei b/src/Relude_List.rei
index 99a1026d..bda60341 100644
--- a/src/Relude_List.rei
+++ b/src/Relude_List.rei
@@ -189,6 +189,7 @@ let removeFirst:
 let removeEach:
   ((module BsAbstract.Interface.EQ with type t = 'a), 'a, list('a)) =>
   list('a);
+let replaceAt: (int, 'a, list('a)) => list('a);
 let scanLeft: (('b, 'a) => 'b, 'b, list('a)) => list('b);
 let scanRight: (('a, 'b) => 'b, 'b, list('a)) => list('b);
 module ListEqExtensions = Relude_Extensions_List.ListEqExtensions;
diff --git a/src/array/Relude_Array_Base.re b/src/array/Relude_Array_Base.re
index 86560998..d28c86ab 100644
--- a/src/array/Relude_Array_Base.re
+++ b/src/array/Relude_Array_Base.re
@@ -260,6 +260,18 @@ let removeEach =
   removeEachBy(EqA.eq, x, xs);
 };
 
+let replaceAt: 'a. (int, 'a, array('a)) => array('a) =
+  (targetIndex, newX, xs) => {
+    xs
+    |> mapWithIndex((x, currentIndex) =>
+         if (currentIndex == targetIndex) {
+           newX;
+         } else {
+           x;
+         }
+       );
+  };
+
 // TODO: scans should come from a typeclass/extension
 let scanLeft: (('b, 'a) => 'b, 'b, array('a)) => array('b) =
   (f, init, xs) =>
@@ -285,4 +297,4 @@ let scanRight: (('a, 'b) => 'b, 'b, array('a)) => array('b) =
         (init, [||]),
         xs,
       ),
-    );
+    );
\ No newline at end of file
diff --git a/src/list/Relude_List_Base.re b/src/list/Relude_List_Base.re
index 701bc6be..11d7054a 100644
--- a/src/list/Relude_List_Base.re
+++ b/src/list/Relude_List_Base.re
@@ -255,6 +255,18 @@ let removeEach =
   removeEachBy(EqA.eq, x, xs);
 };
 
+let replaceAt: 'a. (int, 'a, list('a)) => list('a) =
+  (targetIndex, newX, xs) => {
+    xs
+    |> mapWithIndex((x, currentIndex) =>
+         if (currentIndex == targetIndex) {
+           newX;
+         } else {
+           x;
+         }
+       );
+  };
+
 // TODO: scans come from TraversableExtensions
 let scanLeft: (('b, 'a) => 'b, 'b, list('a)) => list('b) =
   (f, init, xs) =>
@@ -279,4 +291,4 @@ let scanRight: (('a, 'b) => 'b, 'b, list('a)) => list('b) =
       (init, []),
       xs,
     )
-    |> snd;
+    |> snd;
\ No newline at end of file