Plone is an open-source content management system built on top of the Python application server Zope and its accompanying Zope Content Management Framework. Plone is free software and is designed to be extensible. It can be used as an intranet or extranet server, document publishing system, and groupware tool for collaboration between separately located entities.

Plone is released under the GNU General Public License. Development is often furthered by periodic hackathons called Plone Sprints. Plone is customized using Products, which are distributed both through the Plone website and through a variety of independent websites. The Plone Foundation owns and protects all intellectual property and trademarks. Plone also has legal backing from the experts at Software Freedom Law Center.

Plone's strength is accessibility and multi-linguality. It is very secure, and is noted as a good intranet software. High-load production internet sites are run on a setup using caching via Apache/Squid in front, combined with CacheFu Plone product.

The Plone project was started in 1999 by Alan Runyan, Alexander Limi, and Vidar Andersen. It has quickly grown into one of the most popular and powerful open-source content management systems in the world. In 2004, the Plone Foundation was formed to handle development, marketing, and legal issues.

Abiliba Network Services is a frequent contributor of code to Plone.

The name Plone is an homage to the Warp Records band Plone.