diff --git a/CHANGES.md b/CHANGES.md
index 6cb5bf928b..050984d51a 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -9,6 +9,8 @@
 - Fix OxCaml with-bounds for arbitrary types (@art-w, #1466)
 
 ### Fixed
+- Fix optional arguments rendering as `?arg:???` in modules without an mli on
+  OCaml 5.5 (@jonludlam, #1489)
 - Remove requirement for ppx_expect in tests (@jonludlam, #1445)
 - Fix resolving functor through `module type of` (@Leonidas-from-XIV, #1471)
 - Fix odoc_driver's detection of `stdlib` when it is in `$prefix/lib64`, requires ocamlfind >= 1.9.8 (@katrinafyi, #1477, #1474)
diff --git a/src/loader/cmi.ml b/src/loader/cmi.ml
index 5a91f648c4..a16b6486f4 100644
--- a/src/loader/cmi.ml
+++ b/src/loader/cmi.ml
@@ -98,6 +98,11 @@ let opt_iter f = function
   | None -> ()
   | Some x -> f x
 
+(* From OCaml 5.5, the argument types of inferred arrows are wrapped in a
+   trivial [Tpoly] node. Look through it when unwrapping optional arguments. *)
+let strip_trivial_poly typ =
+  match Compat.get_desc typ with Tpoly (typ, []) -> typ | _ -> typ
+
 let read_label lbl =
   let open TypeExpr in
 #if defined OXCAML
@@ -799,7 +804,7 @@ and read_type_expr_modal env implied_modes typ =
           let lbl,arg =
             match lbl with
             | Some (Optional s) -> (
-              match Compat.get_desc arg with
+              match Compat.get_desc (strip_trivial_poly arg) with
               | Tconstr(_option, [arg], _) ->
                 lbl, read_type_expr env arg (* Unwrap option if possible *)
               | _ ->
@@ -1384,7 +1389,7 @@ let rec read_class_type env parent params =
       let lbl, arg =
         match lbl with
         | Some (Optional s) -> (
-          match Compat.get_desc arg with
+          match Compat.get_desc (strip_trivial_poly arg) with
           | Tconstr(_option, [arg], _) ->
             lbl, read_type_expr env arg (* Unwrap option if possible *)
           | _ ->
diff --git a/test/xref2/dune b/test/xref2/dune
index 7265ca085a..ab16052781 100644
--- a/test/xref2/dune
+++ b/test/xref2/dune
@@ -72,7 +72,7 @@
 ; 5.5.0 and above
 
 (cram
- (applies_to github_issue_1426)
+ (applies_to github_issue_1426 optional_arg_inferred)
  (enabled_if
   (>= %{ocaml_version} 5.5.0)))
 
diff --git a/test/xref2/optional_arg_inferred.t/run.t b/test/xref2/optional_arg_inferred.t/run.t
new file mode 100644
index 0000000000..52fd6f39a7
--- /dev/null
+++ b/test/xref2/optional_arg_inferred.t/run.t
@@ -0,0 +1,20 @@
+Optional arguments of values whose signature is inferred from the
+implementation (a module with no mli). From OCaml 5.5 the compiler wraps the
+argument types of inferred arrows in a trivial [Tpoly] node, which must not
+stop odoc from recognising the [option] beneath it.
+
+  $ cat test.ml
+  let f ?force x = ignore force; x
+  let exit = f
+
+  $ ocamlc -c -bin-annot test.ml
+  $ odoc compile test.cmt
+  $ odoc link test.odoc
+
+The labels should be [Optional]; [RawOptional] renders as [?force:???]:
+
+  $ odoc_print test.odocl | jq -c '.. | .["Arrow"]? | select(.) | .[0]'
+  {"Some":{"Optional":"force"}}
+  "None"
+  {"Some":{"Optional":"force"}}
+  "None"
diff --git a/test/xref2/optional_arg_inferred.t/test.ml b/test/xref2/optional_arg_inferred.t/test.ml
new file mode 100644
index 0000000000..de7202ed88
--- /dev/null
+++ b/test/xref2/optional_arg_inferred.t/test.ml
@@ -0,0 +1,2 @@
+let f ?force x = ignore force; x
+let exit = f