diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3021af3e..92d925c8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@
 
 ### Fixed
 
+- [Router] Compose and restore `SCRIPT_NAME` correctly for mounted Rack apps.
 - [OpenAPI] Fix schema registry key collisions for Blueprinter classes referenced under different views.
 
 ## [1.26.0] - 2026-07-07
diff --git a/lib/rage/router/backend.rb b/lib/rage/router/backend.rb
index 9e59f0d6..4b27d7f6 100644
--- a/lib/rage/router/backend.rb
+++ b/lib/rage/router/backend.rb
@@ -35,13 +35,17 @@ def mount(path, handler, methods)
       # by the time the app is called, `rack.input` is already consumed in `Rage::ParamsParser`
       env["rack.input"].rewind
 
-      env["SCRIPT_NAME"] = path
-      sub_path = env["PATH_INFO"].delete_prefix!(path)
-      env["PATH_INFO"] = "/" if sub_path == ""
+      script_name = env["SCRIPT_NAME"]
+      path_info = env["PATH_INFO"]
+
+      env["SCRIPT_NAME"] = "#{script_name}#{path}"
+      sub_path = path_info.delete_prefix(path)
+      env["PATH_INFO"] = sub_path == "" ? "/" : sub_path
 
       handler.call(env)
     ensure
-      env["PATH_INFO"] = "#{env["SCRIPT_NAME"]}#{sub_path}"
+      env["SCRIPT_NAME"] = script_name
+      env["PATH_INFO"] = path_info
     end
 
     methods.each do |method|
diff --git a/spec/router/mount_spec.rb b/spec/router/mount_spec.rb
index a324bd73..2eda5031 100644
--- a/spec/router/mount_spec.rb
+++ b/spec/router/mount_spec.rb
@@ -28,6 +28,30 @@
     expect(result).to eq("/rack_app")
   end
 
+  it "composes and restores script name for mounted apps" do
+    seen = nil
+
+    router.mount("/rack_app", ->(env) {
+      seen = { script_name: env["SCRIPT_NAME"], path_info: env["PATH_INFO"] }
+      :rack_app_response
+    }, %w(GET))
+
+    env = {
+      "REQUEST_METHOD" => "GET",
+      "PATH_INFO" => "/rack_app/index",
+      "SCRIPT_NAME" => "/outer",
+      "rack.input" => StringIO.new
+    }
+
+    handler = router.lookup(env)
+    result = handler[:handler].call(env, handler[:params])
+
+    expect(result).to eq(:rack_app_response)
+    expect(seen).to eq({ script_name: "/outer/rack_app", path_info: "/index" })
+    expect(env["SCRIPT_NAME"]).to eq("/outer")
+    expect(env["PATH_INFO"]).to eq("/rack_app/index")
+  end
+
   it "updates path info" do
     router.mount("/rack_app", ->(env) { env["PATH_INFO"] }, %w(GET))