diff --git a/NEWS.md b/NEWS.md
index ff0bf012992..fb926d4c747 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -66,6 +66,8 @@ Note: We're only listing outstanding class updates.
 
 * Module
 
+    * `Module#descendants` is added.  It returns an array of classes and
+      modules that have the receiver in their ancestors.  [[Feature #9779]]
     * `Module#ruby2_keywords` and top-level `ruby2_keywords` are
       deprecated and will be removed in Ruby 4.4. [[Feature #22205]]
 
@@ -298,6 +300,7 @@ A lot of work has gone into making Ractors more stable, performant, and usable.
 ## JIT
 
 [Feature #8948]: https://bugs.ruby-lang.org/issues/8948
+[Feature #9779]: https://bugs.ruby-lang.org/issues/9779
 [Feature #15330]: https://bugs.ruby-lang.org/issues/15330
 [Feature #20163]: https://bugs.ruby-lang.org/issues/20163
 [Feature #21390]: https://bugs.ruby-lang.org/issues/21390
diff --git a/class.c b/class.c
index f591eb9a2e6..79817af97d5 100644
--- a/class.c
+++ b/class.c
@@ -1005,9 +1005,7 @@ rb_mod_init_copy(VALUE clone, VALUE orig)
             rb_class_set_super(prev_clone_p, clone_p);
             prev_clone_p = clone_p;
             RCLASS_SET_CONST_TBL(clone_p, RCLASS_CONST_TBL(p), false);
-            if (RB_TYPE_P(clone, T_CLASS)) {
-                RCLASS_SET_INCLUDER(clone_p, clone);
-            }
+            RCLASS_SET_INCLUDER(clone_p, clone);
             add_subclass = TRUE;
             if (p != RCLASS_ORIGIN(p)) {
                 origin[0] = clone_p;
@@ -2118,6 +2116,115 @@ rb_class_subclasses(VALUE klass)
     return class_descendants(klass, true);
 }
 
+struct descendants_traverse_data
+{
+    VALUE buffer;
+    long count;
+    long maxcount;
+    st_table *visited;
+};
+
+static void module_descendants_recursive(VALUE entry, VALUE v);
+
+static void
+module_descendants_add(VALUE klass, struct descendants_traverse_data *data)
+{
+    // skip entries beyond the estimation to keep the enumeration pass allocation-free
+    if (data->buffer && data->count >= data->maxcount) return;
+
+    if (st_insert(data->visited, (st_data_t)klass, 1)) return; // already visited
+
+    if (data->buffer) {
+        // assumes that this does not cause GC as long as the length does not exceed the capacity
+        rb_ary_push(data->buffer, klass);
+    }
+    data->count++;
+    rb_class_foreach_subclass(klass, module_descendants_recursive, (VALUE)data);
+}
+
+static void
+module_descendants_recursive(VALUE entry, VALUE v)
+{
+    struct descendants_traverse_data *data = (struct descendants_traverse_data *)v;
+
+    if (rb_objspace_garbage_object_p(entry)) return;
+
+    if (RB_TYPE_P(entry, T_ICLASS)) {
+        // resolve the ICLASS to the including class or module;
+        // refinement ICLASSes have no includer
+        VALUE includer = RCLASS_INCLUDER(entry);
+        while (includer && RB_TYPE_P(includer, T_ICLASS)) {
+            includer = RCLASS_INCLUDER(includer);
+        }
+        if (!includer || UNDEF_P(includer)) return;
+        if (rb_objspace_garbage_object_p(includer)) return;
+        if (RCLASS_SINGLETON_P(includer)) return; // e.g. Object#extend
+        module_descendants_add(includer, data);
+    }
+    else {
+        if (RCLASS_SINGLETON_P(entry)) return;
+        module_descendants_add(entry, data);
+    }
+}
+
+/*
+ *  call-seq:
+ *     descendants -> array
+ *
+ *  Returns an array of classes and modules that have the receiver in
+ *  their ancestors. This is the inverse of Module#ancestors:
+ *  +x.descendants.include?(y)+ holds if and only if
+ *  +y.ancestors.include?(x)+ holds, except that the receiver itself,
+ *  singleton classes, and refinements are never included.
+ *  The order of the returned array is not defined.
+ *
+ *     module A; end
+ *     module B; include A; end
+ *     class C; include B; end
+ *     class D < C; end
+ *
+ *     A.descendants    #=> [B, C, D]
+ *     B.descendants    #=> [C, D]
+ *     C.descendants    #=> [D]
+ *
+ *  Note that the receiver does not hold references to its descendants
+ *  and doesn't prevent them from being garbage collected. This means
+ *  that a descendant might disappear from the result when all
+ *  references to it are dropped, depending on whether garbage
+ *  collector was run.
+ */
+
+VALUE
+rb_mod_descendants(VALUE mod)
+{
+    struct descendants_traverse_data data = { Qfalse, 0, -1, NULL };
+
+    // estimate the count of descendants
+    data.visited = st_init_numtable();
+    st_insert(data.visited, (st_data_t)mod, 1); // exclude the receiver
+    rb_class_foreach_subclass(mod, module_descendants_recursive, (VALUE)&data);
+    st_free_table(data.visited);
+
+    // the following allocation may cause GC which may change the number of descendants
+    data.buffer = rb_ary_new_capa(data.count);
+    data.maxcount = data.count;
+    data.count = 0;
+    // pre-sized so that st_insert() does not cause GC during the enumeration
+    data.visited = st_init_numtable_with_size(data.maxcount + 1);
+    st_insert(data.visited, (st_data_t)mod, 1); // exclude the receiver
+
+    size_t gc_count = rb_gc_count();
+
+    rb_class_foreach_subclass(mod, module_descendants_recursive, (VALUE)&data);
+
+    if (gc_count != rb_gc_count()) {
+        rb_bug("GC must not occur during the subclass iteration of Module#descendants");
+    }
+    st_free_table(data.visited);
+
+    return data.buffer;
+}
+
 /*
  *  call-seq:
  *     attached_object -> object
diff --git a/include/ruby/internal/intern/class.h b/include/ruby/internal/intern/class.h
index 63a2bc09017..1e6822c3eaf 100644
--- a/include/ruby/internal/intern/class.h
+++ b/include/ruby/internal/intern/class.h
@@ -174,6 +174,18 @@ VALUE rb_mod_include_p(VALUE child, VALUE parent);
  */
 VALUE rb_mod_ancestors(VALUE mod);
 
+/**
+ * Queries the module's descendants.  This routine gathers classes and modules
+ * that have the given module in their ancestors.  The returned array includes
+ * neither the given module itself nor singleton classes.
+ *
+ * @param[in]  mod  A module or a class.
+ * @return     An array of classes and modules whose `ancestors` include `mod`.
+ *
+ * @internal
+ */
+VALUE rb_mod_descendants(VALUE mod);
+
 /**
  * Queries the class's direct descendants. This  routine gathers classes that are
  * direct subclasses of the given class,
diff --git a/object.c b/object.c
index 2eb958fb151..bd53644a1c3 100644
--- a/object.c
+++ b/object.c
@@ -4632,6 +4632,7 @@ InitVM_Object(void)
     rb_define_method(rb_cModule, "name", rb_mod_name, 0);  /* in variable.c */
     rb_define_method(rb_cModule, "set_temporary_name", rb_mod_set_temporary_name, 1);  /* in variable.c */
     rb_define_method(rb_cModule, "ancestors", rb_mod_ancestors, 0); /* in class.c */
+    rb_define_method(rb_cModule, "descendants", rb_mod_descendants, 0); /* in class.c */
 
     rb_define_method(rb_cModule, "attr", rb_mod_attr, -1);
     rb_define_method(rb_cModule, "attr_reader", rb_mod_attr_reader, -1);
diff --git a/spec/ruby/core/module/descendants_spec.rb b/spec/ruby/core/module/descendants_spec.rb
new file mode 100644
index 00000000000..779660b933b
--- /dev/null
+++ b/spec/ruby/core/module/descendants_spec.rb
@@ -0,0 +1,137 @@
+require_relative '../../spec_helper'
+require_relative 'fixtures/classes'
+
+ruby_version_is "4.1" do
+  describe "Module#descendants" do
+    it "returns a list of classes inheriting from self" do
+      parent = Class.new
+      child = Class.new(parent)
+      grandchild = Class.new(child)
+
+      assert_descendants(parent, [child, grandchild])
+      assert_descendants(child, [grandchild])
+      assert_descendants(grandchild, [])
+    end
+
+    it "returns a list of modules and classes including self" do
+      mod = Module.new
+      including_mod = Module.new { include mod }
+      klass = Class.new { include including_mod }
+      subclass = Class.new(klass)
+
+      assert_descendants(mod, [including_mod, klass, subclass])
+      assert_descendants(including_mod, [klass, subclass])
+    end
+
+    it "returns modules and classes prepending self" do
+      mod = Module.new
+      prepending_mod = Module.new { prepend mod }
+      klass = Class.new { prepend mod }
+
+      assert_descendants(mod, [prepending_mod, klass])
+    end
+
+    it "returns modules and classes including self after prepending another module" do
+      prepended = Module.new
+      mod = Module.new
+      klass = Class.new { prepend prepended; include mod }
+      including_mod = Module.new { prepend prepended; include mod }
+
+      assert_descendants(mod, [klass, including_mod])
+    end
+
+    it "does not return the internal nodes created by prepend" do
+      parent = Class.new { prepend Module.new }
+      child = Class.new(parent) { prepend Module.new }
+
+      assert_descendants(parent, [child])
+    end
+
+    it "does not return modules and classes including a clone of self" do
+      mod = Module.new
+      clone = mod.clone
+      klass = Class.new { include clone }
+
+      mod.descendants.should_not.include?(klass)
+      clone.descendants.should.include?(klass)
+    end
+
+    it "returns modules and classes into which self is included later" do
+      mod = Module.new
+      including_mod = Module.new
+      klass = Class.new { include including_mod }
+      including_mod.include(mod)
+
+      assert_descendants(mod, [including_mod, klass])
+    end
+
+    it "does not return self" do
+      mod = Module.new
+      Class.new { include mod }
+
+      mod.descendants.should_not.include?(mod)
+      ModuleSpecs::Parent.descendants.should_not.include?(ModuleSpecs::Parent)
+    end
+
+    it "returns an empty Array if there is no descendant" do
+      Module.new.descendants.should == []
+      Class.new.descendants.should == []
+    end
+
+    it "does not return singleton classes" do
+      mod = Module.new
+      obj = Object.new
+      obj.extend(mod)
+      klass = Class.new { extend mod }
+
+      mod.descendants.should_not.include?(obj.singleton_class)
+      mod.descendants.should_not.include?(klass.singleton_class)
+      ModuleSpecs::Internal.descendants.should_not.include?(ModuleSpecs::Child.singleton_class)
+    end
+
+    it "does not return refinements" do
+      klass = Class.new
+      refinement = nil
+      Module.new do
+        refinement = refine(klass) do
+          def refined_method; end
+        end
+      end
+
+      klass.descendants.should_not.include?(refinement)
+      refinement.descendants.should_not.include?(klass)
+    end
+
+    it "has 1 entry per module or class" do
+      mod = Module.new
+      including_mod = Module.new { include mod }
+      Class.new do
+        include mod
+        include including_mod
+      end
+
+      descendants = mod.descendants
+      descendants.should == descendants.uniq
+
+      descendants = ModuleSpecs::Parent.descendants
+      descendants.should == descendants.uniq
+    end
+
+    it "returns the modules and classes whose ancestors include self" do
+      mods = [ModuleSpecs::Basic, ModuleSpecs::Super, ModuleSpecs::Parent,
+              ModuleSpecs::Child, ModuleSpecs::Child2, ModuleSpecs::Grandchild]
+
+      mods.each do |mod|
+        descendants = mod.descendants
+        mods.each do |other|
+          next if mod.equal?(other)
+          descendants.include?(other).should == other.ancestors.include?(mod)
+        end
+      end
+    end
+
+    def assert_descendants(mod, descendants)
+      mod.descendants.sort_by(&:object_id).should == descendants.sort_by(&:object_id)
+    end
+  end
+end
diff --git a/test/ruby/test_module.rb b/test/ruby/test_module.rb
index ad83d098236..c011da4a5c5 100644
--- a/test/ruby/test_module.rb
+++ b/test/ruby/test_module.rb
@@ -221,6 +221,83 @@ class TestModule < Test::Unit::TestCase
     assert_equal([String, Comparable, Object, Kernel, BasicObject], String.ancestors - mixins)
   end
 
+  def test_descendants
+    a = Module.new
+    b = Module.new { include a }
+    c = Class.new { include b }
+    d = Class.new(c)
+
+    order = [a, b, c, d]
+    assert_equal([b, c, d], a.descendants.sort_by {|m| order.index(m)})
+    assert_equal([c, d], b.descendants.sort_by {|m| order.index(m)})
+    assert_equal([d], c.descendants)
+    assert_equal([], d.descendants)
+
+    # prepend
+    pre = Module.new
+    e = Class.new { prepend pre }
+    assert_include(pre.descendants, e)
+
+    # diamond inclusion does not produce duplicates
+    f = Class.new { include a; include b }
+    descendants = a.descendants
+    assert_equal(descendants.uniq, descendants)
+    assert_include(descendants, f)
+
+    # include into a module propagates to existing includers
+    g = Module.new
+    h = Module.new
+    i = Class.new { include h }
+    h.include(g)
+    assert_include(g.descendants, h)
+    assert_include(g.descendants, i)
+
+    # singleton classes are excluded, even via Object#extend
+    obj = Object.new
+    obj.extend(a)
+    assert_not_include(a.descendants, obj.singleton_class)
+
+    # duality with Module#ancestors, except that the receiver is excluded
+    all = [a, b, c, d, pre, e, f, g, h, i]
+    all.each do |x|
+      dx = x.descendants
+      assert_not_include(dx, x)
+      all.each do |y|
+        next if x == y
+        assert_equal(y.ancestors.include?(x), dx.include?(y),
+                     "#{x.inspect} vs #{y.inspect}")
+      end
+    end
+  end
+
+  def test_descendants_gc
+    a = Module.new
+    Class.new { include a }
+    Module.new { include a }
+    GC.start
+    descendants = a.descendants
+    assert_not_include(descendants, a)
+    # the anonymous descendants above may or may not have been collected
+    # already; what must hold is that no dead object is returned
+    descendants.each {|m| assert_kind_of(Module, m)}
+  end
+
+  def test_descendants_refinement
+    assert_separately([], <<-"end;")
+      class Target; def t; end; end
+      module M; end
+      refinement = nil
+      Module.new do
+        refinement = refine(Target) do
+          def t2; end
+        end
+      end
+      assert_equal([], Target.descendants)
+      assert_not_include(M.descendants, Target)
+      assert_not_include(Target.descendants, refinement)
+    end;
+  end
+
   CLASS_EVAL = 2
   @@class_eval = 'b'