Ballylahan Castle, Medieval castle in County Mayo, Ireland.

Ballylahan Castle is a ruined tower house and National Monument in County Mayo, standing on the south bank of the River Moy. Its main entrance, in the east, was flanked by two circular towers, of which only one survives. The entrance leads into an irregular hexagonal bawn with traces of buildings against the western wall and base-batter at the northwest corner.

Jordan de Exeter, Sheriff of Connacht, built this fortress in 1260 as part of the Norman expansion into Ireland. The stronghold suffered major destruction in 1316 when forces led by King Fedlim Ó Conchobair attacked the site.

The castle was the seat of the Mac Siúrtáin (MacJordans), the Gaelicised family descended from Jordan de Exeter, and its Irish name Caisleán Átha Leathain means "castle of the broad ford". The name records a crossing of the River Moy beside the site.

The ruin stands 2.3 km northeast of Strade, on the south bank of the River Moy. The main entrance was in the east, flanked by two circular towers of which only one survives. The bawn is irregularly hexagonal, with base-batter at the northwest corner.

Local tradition speaks of a roughly 2-kilometer tunnel that once connected this castle to a nearby friary in the village of Strade. Whether this underground passage ever actually existed remains a mystery that visitors often wonder about when exploring the site.