Castlebar (Irish:Caisleán an Bharraigh, meaning "Barry's Castle") is the county town of County Mayo, Ireland. It is in the middle of the county and is its largest town by population.

A campus of Galway-Mayo Institute of Technology and the Country Life section of the National Museum of Ireland are two important local amenities. The town is linked by railway to Dublin, Westport and Ballina. The main route by road is the N5. The town is surrounded by several villages, including Ballintubber. Its economy is primarily service-based.