Welches Dam is a former civil parish in Cambridgeshire around 5 miles (8 km) to the north west of Ely. It covered an area of 2,355 acres (953 ha). Within the parish boundaries were the hamlet of the same name and the settlement of Purls Bridge. Welches Dam is the site of the visitor centre for the RSPB Ouse Washes reserve.
The history of the parish is tied to that of The Fens and the battle to drain it in the first half of the 17th century. The original principal drainage channel for the area, the Old Bedford River, runs through the parish.
The parish's name derives from Edward Welsh who built a dam across the Bedford River. Welsh was an employee of the Adventurers, the 17th century entrepreneurs who invested in the results of Vermuyden's efforts to drain the fens.
Fortrey's Hall, the 17th century ancestral seat of the Fortrey family lies in the parish.
The mission church of St Eanswythe was built in 1909.