Spelga Reservoir is a reservoir in the townland of Spelga (Irish: Speilgeach) in the Mourne Mountains of County Down, Northern Ireland. It was formed by the Spelga Dam and sits at over 1,200 ft above sea level.
The reservoir and dam were built as part of a project that had origins in the idea of supplying water to Belfast, in 1894. The Spelga Reservoir itself was developed between 1953 and 1957.
While the reservoir has not been stocked with fish for some years, it is home to wild trout. The Portadown and Banbridge areas receive their water supply from it.