Area of municipality Trebinje is placed in the southernmost part of Republic of Srpska and BiH. It is the hinterland of south Jadran or the hinterland of Dubrovnik’s and Herceg Nova’s littoral.
Municipality Trebinje covers the area of 904 km² and it is considered to be of medial size when compared to the size of other municipalities in Republic of Srpska.
 
In the north, the municipality is bounded by municipality Bileca, in the northwest with municipality Ljubinje, in the south and southwest with municipality Ravno (Federation BiH), in the south and southeast with municipality Dubrovnik (Croatia Republic) and municipality Herceg Novi (Montenegro), and in the east with municipality Niksic (Montenegro).