Elmer is a borough in Salem County in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 1,347, a decrease of 48 (−3.4%) from the 2010 census count of 1,395, which in turn reflected an increase of 11 (+0.8%) from the 1,384 counted in the 2000 census. It is the home of the annual Appel Farm Arts and Music Festival, which celebrated its 23rd year in 2012.

Salem County is the westernmost county in the U.S. state of New Jersey. Its western boundary is formed by the Delaware River and its eastern terminus is the Delaware Memorial Bridge, which connects the county with New Castle, Delaware. Its county seat is Salem. The county is part of the South Jersey region of the state.