diff --git a/stdlib/bytesLabels.mli b/stdlib/bytesLabels.mli
index 23ccaf391..b92e76d09 100644
--- a/stdlib/bytesLabels.mli
+++ b/stdlib/bytesLabels.mli
@@ -112,6 +112,11 @@ val map : f:(char -> char) -> bytes -> bytes
     stores the resulting bytes in a new sequence that is returned as
     the result. *)
 
+val mapi : (int -> char -> char) -> bytes -> bytes
+(** [mapi f s] calls [f] with each character of [s] and its
+    index (in increasing index order) and stores the resulting bytes
+    in a new sequence that is returned as the result. *)
+
 val trim : bytes -> bytes
 (** Return a copy of the argument, without leading and trailing
     whitespace. The bytes regarded as whitespace are the ASCII
diff --git a/stdlib/stringLabels.mli b/stdlib/stringLabels.mli
index dcef6db03..1cf5d51ed 100644
--- a/stdlib/stringLabels.mli
+++ b/stdlib/stringLabels.mli
@@ -104,6 +104,12 @@ val map : f:(char -> char) -> string -> string
    is returned.
    @since 4.00.0 *)
 
+val mapi : f:(int -> char -> char) -> string -> string
+(** [String.mapi f s] calls [f] with each character of [s] and its
+    index (in increasing index order) and stores the results in a new
+    string that is returned.
+    @since 4.02.0 *)
+
 val trim : string -> string
 (** Return a copy of the argument, without leading and trailing
    whitespace.  The characters regarded as whitespace are: [' '],