Composer is an application-level package manager for the PHP programming language that provides a standard format for managing dependencies of PHP software and required libraries. It was developed by Nils Adermann and Jordi Boggiano, who continue to manage the project. They began development in April 2011 and first released it on March 1, 2012. Composer is strongly inspired by Node.js's "npm" and Ruby's "bundler". The project's dependency solving algorithm started out as a PHP-based port of openSUSE's libzypp satsolver.

Composer runs from the command line and installs dependencies (e.g. libraries) for an application. It also allows users to install PHP applications that are available on "Packagist" which is its main repository containing available packages. It also provides autoload capabilities for libraries that specify autoload information to ease usage of third-party code.