Mainstone is a small village and civil parish in southwest Shropshire, England, near the border with Powys, Wales.

It lies in the Clun Forest, a large remote and very rural area which is semi-forested and rather hilly.

The Church of England parish church, dedicated to St John the Baptist, lies in the hamlet of Churchtown (alternatively written as "Church Town") a mile west of Mainstone, though the village itself contains a Primitive Methodist chapel, built in 1892 and still in use in the mid-2000s.

Offa's Dyke also cuts north–south through the area, bisecting the hamlet of Churchtown.

The long-distance footpaths "Shropshire Way" and "Offa's Dyke Path" pass through the area and join just outside Churchtown.