The Brent Reservoir (popularly called the Welsh Harp) is a reservoir in North West London. It straddles the boundary between the boroughs of Brent and Barnet and is owned by the Canal & River Trust. The reservoir takes its informal name from a public house called The Welsh Harp, which stood nearby until the early 1970s. It is a 68.6-hectare (170-acre) biological Site of Special Scientific Interest (SSSI), the only SSSI in either borough and among more than 30 SSSIs in Greater London.

The reservoir is fed by the Silk Stream and the River Brent. Its main outflow is the River Brent. Its smaller outflow is a feeder channel to the canal system. It holds an estimated 1,600,000 m³ (57,000,000 cu ft). In 1994 when the reservoir was drained more than 3,000 kg (6,700 lb) of fish were captured, 95% of which were roach. However, fishing is prohibited.

The reservoir has a sailing centre, home to Welsh Harp Sailing Club, Wembley Sailing Club, the Sea Cadets, and the University of London Sailing club. In 1960, it also hosted the Women's European Rowing Championships. The reservoir also hosts the Phoenix canoe club and outdoor centre.