diff --git a/sledge/nonstdlib/option.ml b/sledge/nonstdlib/option.ml
index dce244872..60c96364b 100644
--- a/sledge/nonstdlib/option.ml
+++ b/sledge/nonstdlib/option.ml
@@ -16,6 +16,7 @@ let pp fmt pp_elt fs = function
   | None -> ()
 
 let map_or xo ~default ~f = map_or ~default f xo
+let flat_map xo ~f = flat_map f xo
 let iter xo ~f = iter f xo
 let exists xo ~f = exists f xo
 let for_all xo ~f = for_all f xo
diff --git a/sledge/nonstdlib/option.mli b/sledge/nonstdlib/option.mli
index 8c12075bc..e4d08d032 100644
--- a/sledge/nonstdlib/option.mli
+++ b/sledge/nonstdlib/option.mli
@@ -17,6 +17,7 @@ include Monad_intf.S with type 'a t := 'a t
 
 val pp : ('a_pp -> 'a -> unit, unit) fmt -> 'a_pp -> 'a option pp
 val map_or : 'a t -> default:'b -> f:('a -> 'b) -> 'b
+val flat_map : 'a t -> f:('a -> 'b t) -> 'b t
 val iter : 'a t -> f:('a -> unit) -> unit
 val exists : 'a t -> f:('a -> bool) -> bool
 val for_all : 'a t -> f:('a -> bool) -> bool