The River Farset (An Fhearsaid or Abhainn na Feirste in Irish) is a river in Belfast, Northern Ireland, which gave the city its name. It is a late tributary of the River Lagan.
The Farset rises above Ballysillan on Squire's Hill, flowing past the medieval parish church at Shankill, through Millfield and under High Street, giving that street a curving appearance, entering the Lagan to the east of the Albert Clock and High Street. It is on the County Antrim side of the Lagan and its entry to the Lagan is close to its outflow into Belfast Lough. The Farset is now contained within a tunnel under Belfast's High Street; a tunnel supposedly big enough to take a bus.