Seaside Park is a borough in Ocean County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 1,579, reflecting a decline of 684 from the 2,263 counted in the 2000 Census, which had in turn increased by 392 from the 1,871 counted in the 1990 Census. Seaside Park is situated on the Barnegat Peninsula, a long, narrow barrier peninsula that separates Barnegat Bay from the Atlantic Ocean.