Legion is a computer software system variously classified as a distributed operating system, a peer-to-peer system, metacomputing software, or middleware. It is an object-based system designed to provide secure, transparent access to large numbers of machines. The project was funded by the National Science Foundation and other funding agencies, and was mostly done at the University of Virginia.

Legion is the successor to Hydra, developed to run on the C.mmp hardware system developed at Carnegie-Mellon University in the late 1960s.

One of the slogans of the Legion project is "mechanism, not policy!"

Legion is currently being commercialized by Avaki, Inc. Their website is: