diff --git a/CHANGELOG.md b/CHANGELOG.md
index 67a55168d3..26607be698 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,7 @@
 - Deprecate `Stdlib_Error` and `Stdlib_Exn` modules in favor of `JsError/JsExn`. https://github.com/rescript-lang/rescript/pull/8404
 - Deprecate `List.has` in favor of `List.some`. https://github.com/rescript-lang/rescript/pull/8807
 - Deprecate `@deriving(abstract)` and `@deriving(jsConverter)`, which will be removed in v13. https://github.com/rescript-lang/rescript/pull/8808
+- Deprecate `@unboxed` on a variant constructor, which is an error in v13: write `@unboxed` on the type. https://github.com/rescript-lang/rescript/pull/9049
 
 # 12.3.1
 
diff --git a/compiler/ml/typedecl.ml b/compiler/ml/typedecl.ml
index 0f44d4595f..53f4ea33a2 100644
--- a/compiler/ml/typedecl.ml
+++ b/compiler/ml/typedecl.ml
@@ -591,8 +591,34 @@ let transl_declaration ~type_record_as_object ~untagged_wfc env sdecl id =
           in
           Some (tcstr, cstr)
       in
+      (* [@unboxed] on a constructor, as written, before the type's
+         attributes are copied to its constructors. v13 rejects it.
+         Constructors copied by a variant spread carry the source type's
+         attributes, already reported at the source. *)
+      let warn_unboxed_constructor scstr =
+        let from_spread =
+          Variant_type_spread.constructor_is_from_spread scstr.pcd_attributes
+        in
+        Ext_list.iter scstr.pcd_attributes (fun ({txt; loc}, _) ->
+            if (not from_spread) && txt = Ast_untagged_variants.untagged then
+              Location.deprecated loc
+                (if Ast_untagged_variants.has_untagged sdecl.ptype_attributes
+                 then
+                   Printf.sprintf
+                     "@unboxed on constructor %s is deprecated and will be an \
+                      error in v13. The type is already @unboxed, so remove it \
+                      from the constructor."
+                     scstr.pcd_name.txt
+                 else
+                   Printf.sprintf
+                     "@unboxed on constructor %s is deprecated and will be an \
+                      error in v13, where @unboxed applies to a whole variant: \
+                      write it on the type, as in `@unboxed type t = ...`."
+                     scstr.pcd_name.txt))
+      in
       let make_cstr scstr =
         Builtin_attributes.warning_scope scstr.pcd_attributes (fun () ->
+            warn_unboxed_constructor scstr;
             make_cstr scstr)
       in
       let tcstrs, cstrs = List.split (List.filter_map make_cstr scstrs) in
diff --git a/tests/build_tests/super_errors/expected/unboxed_on_constructor_deprecated.res.expected b/tests/build_tests/super_errors/expected/unboxed_on_constructor_deprecated.res.expected
new file mode 100644
index 0000000000..2622fc3a2e
--- /dev/null
+++ b/tests/build_tests/super_errors/expected/unboxed_on_constructor_deprecated.res.expected
@@ -0,0 +1,21 @@
+
+  Warning number 3
+  /.../fixtures/unboxed_on_constructor_deprecated.res:1:16-23
+
+  1 │ type mixed = | @unboxed Text(string) | Count(int)
+  2 │ 
+  3 │ @unboxed
+
+  deprecated: @unboxed on constructor Text is deprecated and will be an error in v13, where @unboxed applies to a whole variant: write it on the type, as in `@unboxed type t = ...`.
+
+
+  Warning number 3
+  /.../fixtures/unboxed_on_constructor_deprecated.res:4:16-23
+
+  2 │ 
+  3 │ @unboxed
+  4 │ type whole = | @unboxed Name(string) | Age(int)
+  5 │ 
+  6 │ type spreadMixed = | ...mixed | Flag(bool)
+
+  deprecated: @unboxed on constructor Name is deprecated and will be an error in v13. The type is already @unboxed, so remove it from the constructor.
\ No newline at end of file
diff --git a/tests/build_tests/super_errors/fixtures/unboxed_on_constructor_deprecated.res b/tests/build_tests/super_errors/fixtures/unboxed_on_constructor_deprecated.res
new file mode 100644
index 0000000000..dc8c855b30
--- /dev/null
+++ b/tests/build_tests/super_errors/fixtures/unboxed_on_constructor_deprecated.res
@@ -0,0 +1,12 @@
+type mixed = | @unboxed Text(string) | Count(int)
+
+@unboxed
+type whole = | @unboxed Name(string) | Age(int)
+
+type spreadMixed = | ...mixed | Flag(bool)
+
+@unboxed
+type plain = Plain(string)
+
+@unboxed
+type spreadPlain = | ...plain | Num(int)