The Klingon-Romulan border was the border between the Klingon Empire and the Romulan Star Empire.
Amongst the planets located along the border were Nequencia, on the Romulan side, and Khitomer, on the Klingon side. (ST reference: Star Charts; TOS movie: The Undiscovered Country)
During the Klingon Civil War in early 2367, Captain Jean-Luc Picard and the USS Enterprise-D led a fleet of twenty-three Federation starships and blockaded the border in an attempt to prevent "any external power" from interfering the the war. (TNG episode: "Redemption II")
During an increase in aggression against their neighbors in early 2372, the Klingons attacked a number of Romulan outposts along the border. (DS9 episode: "Hippocratic Oath")