The Geminus Gulf was a region in the Beta Quadrant near the common border between the United Federation of Planets and the Romulan Star Empire. Comprising three sectors of space, the Gulf was sixty light years across and measured about sixty-six thousand cubic parsecs. Sparsely populated with primarily O type stars, at the astrographic center of the Gulf was the Chiaros system, containing the region's only known inhabited planet, Chiaros IV.

Because of treaty provisions, neither the Federation nor the Romulan Empire could expanded into the Gulf, unless first invited by a local species. Hence, Chiaros became of great importance to both powers in the early 2370s.

In violation of the treaty, the Romulans constructed a cloaked starbase near Chiaros, and destroyed the USS Slayton while trying to subvert the Chiarosian government. The Romulan agenda for the Gulf was disrupted by the USS Enterprise-E in 2373. (TNG - S31 novel: Rogue)