Northfield is a city in Atlantic County, New Jersey, United States. As of the United States 2000 Census, the city population was 7,725. Northfield was incorporated as a city by an Act of the New Jersey Legislature on March 21, 1905, from portions of Egg Harbor Township. A portion of Egg Harbor Township was transferred to Northfield in 1931, and then reverted back to Egg Harbor Township in 1933.