Mertola town is situated in the middle of the Guadiana Valley Natural Park in Portugal. It is a protected nature reserve situated in southeastern parts of Portugal in Alentejo region. Ribeira de Oeiras River is a tributary of the Guadiana River. A bridge over Ribeira de Oeiras River connects the road to the western side of the town.
The River Guadiana flows on the eastern side of Mertola. The river is 818 km long, of which 140 km lies within the borders of Portugal. The source of River Guadiana lies in Castilla-La Mancha (Spain) then it flows into Portugal and then back to Spain. On the western side of the river, the old town of Mertola sits on a hill.