Pi Fleet was a Star Trek based simming group that existed in the early 2000s. It was created by Ken Gillis and at its height claimed roughly 400 members.

The organization was founded by Ken Gillis in the mid 1990s, as an offshoot of an earlier sim, Federation. The concept of Pi Fleet was originally going to be fleet based, with one person per ship, acting as Captain, in fleet operations. This idea was abandoned, and the organization went into hiatus in 1998. Pi Fleet was restarted in 2000 when the USS Lakota-B broke off from Bravo Fleet, and the Lakota became the flagship of the new organization. The original website for the Lakota can be seen here, at , untouched ever since the ship was retired as a museum to the vessel.

Pi Fleet was a member of the Simming League for many years. After Gillis removed Pi Fleet from the League due to the failure of the League, as he perceived it, the group went on for a few weeks as normal. In Mid-2004, however, Gillis "retired" for the first time from simming and broke the group into multiple groups along it's Task Force lines.

One such organization that formed from the breaking apart of Pi Fleet was United Science Fiction Simulations, which ran up until 2007, under the command of Roberto Winners.