Adobe Flex is a collection of technologies released by Adobe Systems for the development and deployment of cross platform, rich Internet applications based on the proprietary Adobe Flash platform. The initial release in March 2004 by Macromedia included a software development kit, an IDE, and a J2EE integration application known as Flex Data Services. Since Adobe acquired Macromedia in 2005, subsequent releases of Flex no longer require a license for Flex Data Services, which has become a separate product rebranded as LiveCycle Data Services.

In April 2007, Adobe announced plans to open source the Flex 3 SDK. Adobe Flash Player, the runtime on which Flex applications are viewed, and Flex Builder, the IDE used to build Flex applications, remain proprietary.