This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
In 2375, the location of Starbase 375 was labeled on a tactical cartography map that was displayed in the wardroom of Deep Space 9. Starbase 375 was located approximately thirty light years from Cardassia Prime, twenty-five light years from Bajor, and fifty-five light years from Ferenginar. (DS9: "When It Rains...", okudagram)
In 2399, the location of Starbase 375 was labeled in a Federation star chart that was in Fleet Admiral Kirsten Clancy's office at Starfleet Headquarters. The base was in or near Federation space. (PIC: "Maps and Legends")
Following the Second Battle of Deep Space 9 and the subsequent withdrawal of Federation and Klingon forces from Deep Space 9 in late 2373, the USS Defiant, under the command of Captain Benjamin Sisko, was recalled to this starbase for reassignment. Captain Sisko was stationed here as adjutant to Admiral William Ross after Ross relieved him of command of the Defiant. This starbase was also the location of a Jem'Hadar attack ship retrieved by Sisko a year prior from Torga IV. (DS9: "A Time to Stand")
Approximately four months later, Starbase 375 served as gathering point for the Second and Fifth Fleets in preparation for Operation Return, in which combined Federation and Klingon forces managed to retake Deep Space 9. Sisko then returned to his previous post as commander of Deep Space 9. (DS9: "Favor the Bold", "Sacrifice of Angels")
The Star Trek: Deep Space Nine Technical Manual (p. 18) included a directory for Starbase 375 that was created, but seen only in the background of Admiral Ross' office, for "Behind the Lines". It listed several Deep Space Nine production staff members as Starbase 375 personnel, including Ronnie Knox, Ed Cooper, Phil Jacobson, Ralph Johnson, Don Lehman, Mickey Welsh, Murphy Wiltz, Steve Gausche, Jack White, Brian Johnson, Edmond Wright, Derek Garth, Bill Gocke, Mark Overton, Todd Overton, Jim Unsinn, Bob Blackman, Carol Kunz, Phyllis Corcoran, Mary Ellen Bosche, Camille Argus, Jerry Bono, Cleo Severy, Russ English, Myles Abney, Michael Westmore, and Valerie Canamar.
According to Star Trek: Star Charts (p. 47) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"; "The Dominion War: Strategy and Battles, 2373-75"), Starbase 375 was, during the Dominion War, the headquarters for Fifth Fleet.