Abbeycwmhir or Abbey Cwmhir (Welsh: Abaty Cwm Hir, "Abbey in the Long Valley") is a village and community in the valley of the Nant Clywedog in Powys, Wales.

It was a daughter house of Whitland Abbey, and was constructed at the behest of three sons of Madog, the then Prince of southern Powys.

The first community at Dyvanner (Welsh: Ty faenor, "Manor House") failed because of the intervention of Hugh de Mortimer, Earl of Hereford, but in 1176 Rhys ap Gruffydd of Deheubarth re-established the Abbey on land given by Cadwallon ap Madog.

The Abbey was slighted in 1644, during the English Civil War, although some ruins still remain.

There is a memorial stone to Llywelyn ap Gruffudd, the last native Prince of Wales of direct descent, whose body is buried there.