diff --git a/internal/marshal.h b/internal/marshal.h
new file mode 100644
index 00000000000..31b1bb3e21a
--- /dev/null
+++ b/internal/marshal.h
@@ -0,0 +1,17 @@
+#ifndef INTERNAL_MARSHAL_H                               /*-*-C-*-vi:se ft=c:*/
+#define INTERNAL_MARSHAL_H
+/**
+ * @author     Ruby developers <ruby-core@ruby-lang.org>
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ * @brief      Internal header for Marshal.
+ */
+#include "ruby/internal/stdbool.h"    /* for bool */
+#include "ruby/ruby.h"                /* for VALUE */
+
+/* marshal.c */
+bool rb_marshal_compat_lookup(VALUE klass, VALUE (**dumper)(VALUE), VALUE (**loader)(VALUE, VALUE));
+
+#endif /* INTERNAL_MARSHAL_H */
diff --git a/marshal.c b/marshal.c
index ec081e78b9e..b992b98cb3b 100644
--- a/marshal.c
+++ b/marshal.c
@@ -28,6 +28,7 @@
 #include "internal/encoding.h"
 #include "internal/error.h"
 #include "internal/hash.h"
+#include "internal/marshal.h"
 #include "internal/numeric.h"
 #include "internal/object.h"
 #include "internal/re.h"
@@ -155,6 +156,21 @@ rb_marshal_define_compat(VALUE newclass, VALUE oldclass, VALUE (*dumper)(VALUE),
     RB_OBJ_WRITTEN(compat_allocator_tbl_wrapper, Qundef, oldclass);
 }
 
+/* The rb_marshal_define_compat entry for an instance of klass, if any, so the Ractor
+ * courier can dump and load it the way Marshal does. */
+bool
+rb_marshal_compat_lookup(VALUE klass, VALUE (**dumper)(VALUE), VALUE (**loader)(VALUE, VALUE))
+{
+    st_data_t data;
+    rb_alloc_func_t allocator = RCLASS_SINGLETON_P(klass) ? 0 : rb_get_alloc_func(klass);
+    if (!allocator || !st_lookup(compat_allocator_tbl, (st_data_t)allocator, &data)) return false;
+    marshal_compat_t *compat = (marshal_compat_t *)data;
+    if (!compat->dumper || !compat->loader) return false;
+    if (dumper) *dumper = compat->dumper;
+    if (loader) *loader = compat->loader;
+    return true;
+}
+
 struct dump_arg {
     VALUE str, dest;
     st_table *symbols;
diff --git a/ractor.c b/ractor.c
index 20e206de0a4..6c0cecb1b73 100644
--- a/ractor.c
+++ b/ractor.c
@@ -21,6 +21,7 @@
 #include "internal/variable.h"
 #include "eval_intern.h"
 #include "internal/io.h"
+#include "internal/marshal.h"
 #include "internal/ractor.h"
 #include "internal/rational.h"
 #include "internal/re.h"
@@ -2396,6 +2397,7 @@ enum courier_hook {
     COURIER_HOOK_NONE,
     COURIER_HOOK_MARSHAL_DUMP, /* marshal_dump -> alloc + marshal_load */
     COURIER_HOOK_DUMP,         /* _dump        -> klass._load         */
+    COURIER_HOOK_COMPAT,       /* rb_marshal_define_compat dumper -> alloc + loader (Set, Process::Status) */
     COURIER_HOOK_DUMP_DATA,    /* _dump_data   -> alloc + _load_data  */
 };
 
@@ -2405,6 +2407,7 @@ courier_hook_of(VALUE obj)
 {
     if (rb_obj_respond_to(obj, id_marshal_dump, TRUE)) return COURIER_HOOK_MARSHAL_DUMP;
     if (rb_obj_respond_to(obj, id_dump, TRUE)) return COURIER_HOOK_DUMP;
+    if (rb_marshal_compat_lookup(CLASS_OF(obj), NULL, NULL)) return COURIER_HOOK_COMPAT;
     if (BUILTIN_TYPE(obj) == T_DATA && rb_obj_respond_to(obj, id_dump_data, TRUE)) return COURIER_HOOK_DUMP_DATA;
     return COURIER_HOOK_NONE;
 }
@@ -2711,6 +2714,12 @@ courier_capture_hooked(struct courier_build *b, VALUE obj, uint32_t id, enum cou
       case COURIER_HOOK_DUMP_DATA:
         payload = rb_funcallv(obj, id_dump_data, 0, 0);
         break;
+      case COURIER_HOOK_COMPAT: {
+        VALUE (*dumper)(VALUE);
+        rb_marshal_compat_lookup(klass, &dumper, NULL);
+        payload = dumper(obj);
+        break;
+      }
       default:
         rb_bug("courier_capture_hooked: no dump protocol");
     }
@@ -3390,6 +3399,12 @@ rb_ractor_courier_materialize(struct rb_ractor_courier *c)
                 }
                 rb_funcallv(shell, mid, 1, &payload);
                 break;
+              case COURIER_HOOK_COMPAT: {
+                VALUE (*loader)(VALUE, VALUE);
+                rb_marshal_compat_lookup(klass, NULL, &loader);
+                loader(shell, payload);
+                break;
+              }
               default:
                 rb_bug("rb_ractor_courier_materialize: no dump protocol");
             }
diff --git a/test/ruby/test_ractor.rb b/test/ruby/test_ractor.rb
index c1779839c5a..81b40ca2a6d 100644
--- a/test/ruby/test_ractor.rb
+++ b/test/ruby/test_ractor.rb
@@ -271,6 +271,15 @@ class TestRactor < Test::Unit::TestCase
       assert_copy [Time.now, Ractor::Port.new]
       # Dump hooks run after the courier is sized, so enough of them make it grow.
       assert_copy Array.new(2000) { |i| Time.at(i) }
+      # Set has no dump hook of its own; it goes through its rb_marshal_define_compat entry.
+      assert_copy Set.new
+      assert_copy Set[1,2,3]
+      assert_copy Set[+"a", [+"b"], {+"c" => Set[+"d"]}]
+      assert_copy Set[1].compare_by_identity
+      assert_copy Class.new(Set)[1, 2]
+      assert_equal true, echo(Set[1].compare_by_identity).compare_by_identity?
+      set = Set[Ractor::Port.new, Ractor::Port.new]   # Marshal cannot carry a Port
+      assert_equal set.to_a, echo(set).to_a
 
       # Time#_dump keeps these as ivars on the dumped string; Time#== ignores the last two.
       time = Time.at(0, 123456789, :nsec, in: "+09:00")