Gacko is a town and municipality in Republika Srpska, Bosnia and Herzegovina. It is situated in the region of East Herzegovina. As of 2013, the town has a population of 5,784 inhabitants, while the municipality has 8,990 inhabitants.

East Herzegovina is the eastern part of the historical Herzegovina region in Bosnia and Herzegovina, east of the Neretva river, part of the Republika Srpska entity. Major towns are Trebinje, Nevesinje and Bileća, predominantly inhabited by ethnic Serbs. West Herzegovina is the western part, west of the Neretva river, and is today administratively part of the Herzegovina-Neretva Canton and West Herzegovina Canton, predominantly inhabited by ethnic Croats, located in the Federation of B&H entity. The easternmost parts of historical Herzegovina lie in Montenegro, in so-called "Old Herzegovina", which became part of the Principality of Montenegro in 1878. In 1991, local ethnic Serbs of the region declared the territory of SAO East Herzegovina independent and joined other Serb territories into Republika Srpska by 1992. The meso-region has since been named Trebinje Region.