Mertoun is a parish, which lies in the Scottish Borders Council Area, some 4 miles (7 km) east of Melrose and 7 miles (11 km) west of Kelso in the Scottish Borders.
It is located in the old county of Berwickshire, which disappeared following the Local Government (Scotland) Act 1974.
The parish has an area of 26.3 sq. km (10.1 sq. miles). Mertoun has 6 neighbouring parishes; namely Earlston, Makerstoun, Maxton, Melrose, Smailholm and St Boswells. The boundaries of this parish were modified among significant changes recommended by the Boundary Commissioners after the Local Government (Scotland) Act 1889.