The Sudbury Reservoir (2.02 square miles) is an emergency backup Boston metropolitan water reservoir in Massachusetts, located predominantly in Southborough and Marlborough, with small sections in Westborough and Framingham.

To improve the water quality of the local streams, filter beds were constructed adjacent to the reservoir.

In 1947, the obsolete Whitehall, Hopkinton, Ashland and Cochituate reservoirs became state parks; and in 1976, the entire Sudbury System was officially reclassified as an emergency water supply.

[2] In an emergency the Sudbury and Foss reservoirs can be placed into service either as a primary source, as an alternate pass-through for Quabbin/Wachusett reservoir water in the event of a transmission problem blocking the normal transmission pathways, or as a supplemental source in a major drought.

The area's limited public access allows for picnicking, hiking, snowshoeing, cross-country skiing, and shoreline fishing with restrictions.