Castlerea (Irish: An Caisleán Riabhach) is located in the west of County Roscommon, Republic of Ireland. It is the second largest town in the county with a population of 2842 (as of 2006). Translated from Irish, Castlerea can mean Brindled Castle (Caisleán Riabhach) or King's Castle (Caisleán Rí). The town is built on the River Suck and the River Francis (aka River Cloonard), both tributaries of the River Shannon.

The town is located in the West of Co. Roscommon and is approximately 30km from both Roscommon Town and Boyle, 75km from Galway City and 150km from Dublin. The town is situated at the junction of the N60 National Secondary Route and R361 Regional Road. It is also served by the Dublin-West railway line.