Lake Monomonac is an artificial lake that straddles the border between Rindge, New Hampshire, and Winchendon, Massachusetts, in the United States. It was created from a small pond in New Hampshire by the construction of dams on the North Branch of the Millers River, a part of the Connecticut River watershed. Lake Monomonac is 594 acre in size, with 411 acre in New Hampshire and the remaining 183 acre in Massachusetts. The lake has a maximum recorded depth of 22 ft and an average depth of 10 ft. The lake is classified as a warmwater fishery, with observed species including smallmouth and largemouth bass, black crappie, chain pickerel, white perch, pumpkinseed, bluegill, horned pout, and green sunfish.