Mary Elmer Lake is a reservoir on the Cohansey River in Cumberland County, New Jersey, used for water-supply and recreation purposes.

While the dam is within the city limits, most of the lake is within neighboring Hopewell Township.

At normal levels it has a surface area of 22 acres (8.9 ha).

[1] Mary Elmer Lake was originally known as Ireland's Mill Pond.

Construction of the dam was completed in 1924 by the U.S. Army Corps of Engineers.