Monterroso is a municipality in the province of Lugo, within the autonomous community of Galicia in northwestern Spain. It belongs to the comarca of A Ulloa, a historical and geographical district in the interior of the province.
As a Galician municipality, Monterroso forms part of the region's local administrative structure, with the provincial capital of Lugo serving as the nearest major city and administrative centre. The comarca of A Ulloa, to which it belongs, is one of the traditional territorial divisions used for organising services and governance across Galicia.
Galicia as a whole is located in Spain's far northwest, bordered by the Atlantic Ocean and the Bay of Biscay, and is divided into four provinces: A Coruña, Lugo, Ourense, and Pontevedra. Monterroso lies within the province of Lugo, the largest of the four by area.