diff --git a/core/process.rbs b/core/process.rbs
index cfb6ff4c1..45898705a 100644
--- a/core/process.rbs
+++ b/core/process.rbs
@@ -2023,7 +2023,7 @@ class Process::Status < Object
   # *   `false` if the process has completed unsuccessfully and exited.
   # *   `nil` if the process has not exited.
   #
-  def success?: () -> bool
+  def success?: () -> bool?
 
   # <!--
   #   rdoc-file=process.c
diff --git a/test/stdlib/Process_test.rb b/test/stdlib/Process_test.rb
index a0ac7e159..823455ef1 100644
--- a/test/stdlib/Process_test.rb
+++ b/test/stdlib/Process_test.rb
@@ -77,3 +77,30 @@ def spawn_child(script = "exit", **opts)
     Process.spawn(RUBY_EXECUTABLE, "--disable=all", "-e", script, **opts)
   end
 end
+
+class Process_StatusInstanceTest < Test::Unit::TestCase
+  include TestHelper
+
+  testing "::Process::Status"
+
+  def test_success?
+    assert_send_type "() -> true",
+                     status_of("exit"), :success?
+    assert_send_type "() -> false",
+                     status_of("exit 1"), :success?
+
+    # A child killed by a signal has not exited, so `success?` returns `nil`.
+    pid = Process.spawn(RUBY_EXECUTABLE, "--disable=all", "-e", "sleep")
+    Process.kill(:KILL, pid)
+    _, status = Process.wait2(pid)
+    assert_send_type "() -> nil",
+                     status, :success?
+  end
+
+  private
+
+  def status_of(script)
+    _, status = Process.wait2(Process.spawn(RUBY_EXECUTABLE, "--disable=all", "-e", script))
+    status
+  end
+end