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 in size with in New Hampshire and the remaining in Massachusetts. The lake has a maximum recorded depth of and an average depth of.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.