Sinatra is a free and open source software web application library and domain-specific language written in Ruby. It is an alternative to other Ruby web application frameworks such as Ruby on Rails, Merb, Nitro, Camping, and Rango. It is dependent on the Rack web server interface.

Designed and developed by Blake Mizerany, Sinatra is small and flexible. It does not follow the typical model–view–controller pattern used in other frameworks, such as Ruby on Rails. Instead, Sinatra focuses on "quickly creating web-applications in Ruby with minimal effort."

Some notable companies and institutions that use Sinatra include Apple, BBC, the British Government, LinkedIn, Engine Yard, Heroku, GitHub, and Songbird. Travis CI provides much of the financial support for Sinatra's development.

Sinatra was created and open-sourced in 2007.