La Gacilly is a commune in the Morbihan department of Brittany in north-western France. It is located on the right bank of the Aff River, about 15 km (9 mi) north of Redon, on the train line between Vannes and Rennes and about 60 km (37 mi) from both. It extended on 1 January 2017 by merging with former communes of La Chapelle-Gaceline and Glénac.