Phusion Passenger (informally also known as mod_rails and mod_rack among the Ruby community) is a free web server and application server with support for Ruby, Python and Node.js.

Originally designed for web applications built on the Ruby on Rails framework, it was later extended to support arbitrary Ruby web frameworks through the Rack interface.

Phusion Passenger was the "preferred deployment setup" for Ruby on Rails applications in 2016,[6] and has been recommended by the Ruby on Rails authors in 2009.

[7][8] In 2013, in combination with Ruby 2.0, or with the now-discontinued Ruby Enterprise Edition, Phusion Passenger claimed that it was capable of reducing Rails's memory consumption by 33% as well as increasing its performance.

[9] As of Aug 2021 more than 130,000 web sites use Phusion Passenger.