A Dominion War fleet based at Deep Space 9 from 2374, placed under the Klingon General Martok as Supreme Commander on Captain Sisko's recommendation.
The Ninth Fleet is one of the allied fleets of the Dominion War and the one most closely tied to Deep Space 9.
In the episode You Are Cordially Invited, shortly after the station is recovered, General Martok learns that he has been named Supreme Commander of the Ninth Fleet, with his headquarters aboard Deep Space 9. Sisko had put his name forward. Martok grumbles that the honour mostly brings paperwork, but he accepts the post.
From this base Martok directs Klingon and allied operations on the Bajoran front, with Worf serving at his side, in the later stages of the war, before his own rise to Chancellor in Tacking Into the Wind.