Castle Caereinion (Welsh: Castell Caereinion) is a small village and community in Montgomeryshire, Powys, Wales, upon the River Banwy, around 8 miles west of Welshpool, and 4 miles east of Llanfair Caereinion.

It has been suggested that a mound in the churchyard of St Garmon's is the remains of the earthwork castle.

The most recent view is that the mound does not appear motte-like and a survey in 2002 failed to find a surrounding ditch.

Built with slate roof from green squared rubble stone dressed with limestone, it consists of a porch, nave, chancel, and vestry with a broach-spire tower.

St Garmon's was not an important ecclesiastical centre, being classed as a chapel in 1254.