JBoss (pronounced Jay Boss) Application Server is an open source Java EE application server. Implemented 100% in Java, it can be used on any operating system that Java supports.

The core developers were hired by a services company named JBoss Inc. founded by Marc Fleury, the original author of JBoss AS. JBoss Inc. was purchased by RedHat in April 2006 for $420M USD, after a bidding war with Oracle.

JBoss now package the application server, Apache TomCat, and other components used for creating "Enterprise MiddleWare".

Because of its open source nature and ability to be implemented with no licensing fees, it is a popular choice for deploying Java enterprise applications. JBoss profits from providing service for their software stack, and has a worldwide network of partners.