diff --git a/CHANGES.md b/CHANGES.md
index af1a92b..b6f1d58 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,5 +1,9 @@
 ### Unreleased
 
+* [#55](https://github.com/square/rails-auth/pull/55)
+  Allow dynamic injection of credentials.
+  ([@drcapulet])
+
 * [#59](https://github.com/square/rails-auth/pull/59)
   Expose X.509 Subject Alternative Name extension
   in the Rails::Auth::X509::Certificate and provide a convenience
diff --git a/lib/rails/auth/credentials/injector_middleware.rb b/lib/rails/auth/credentials/injector_middleware.rb
index 572de39..44972a0 100644
--- a/lib/rails/auth/credentials/injector_middleware.rb
+++ b/lib/rails/auth/credentials/injector_middleware.rb
@@ -5,7 +5,8 @@ module Auth
     class Credentials
       # A middleware for injecting an arbitrary credentials hash into the Rack environment
       # This is intended for development and testing purposes where you would like to
-      # simulate a given X.509 certificate being used in a request or user logged in
+      # simulate a given X.509 certificate being used in a request or user logged in.
+      # The credentials argument should either be a hash or a proc that returns one.
       class InjectorMiddleware
         def initialize(app, credentials)
           @app = app
@@ -13,7 +14,8 @@ def initialize(app, credentials)
         end
 
         def call(env)
-          env[Rails::Auth::Env::CREDENTIALS_ENV_KEY] = @credentials
+          credentials = @credentials.respond_to?(:call) ? @credentials.call(env) : @credentials
+          env[Rails::Auth::Env::CREDENTIALS_ENV_KEY] = credentials
           @app.call(env)
         end
       end
diff --git a/spec/rails/auth/credentials/injector_middleware_spec.rb b/spec/rails/auth/credentials/injector_middleware_spec.rb
index e419736..46998aa 100644
--- a/spec/rails/auth/credentials/injector_middleware_spec.rb
+++ b/spec/rails/auth/credentials/injector_middleware_spec.rb
@@ -10,4 +10,17 @@
     _response, env = middleware.call(request)
     expect(env[Rails::Auth::Env::CREDENTIALS_ENV_KEY]).to eq credentials
   end
+
+  context "with a proc for credentials" do
+    let(:credentials_proc) { instance_double(Proc) }
+    let(:middleware)       { described_class.new(app, credentials_proc) }
+
+    it "overrides rails-auth credentials in the rack environment" do
+      expect(credentials_proc).to receive(:call).with(request).and_return(credentials)
+
+      _response, env = middleware.call(request)
+
+      expect(env[Rails::Auth::Env::CREDENTIALS_ENV_KEY]).to eq credentials
+    end
+  end
 end