Long Branch is a beachside city in Monmouth County, in the U.S. state of New Jersey. As of the 2020 United States census, the city's populati…Long Branch is a beachside city in Monmouth County, in the U.S. state of New Jersey. As of the 2020 United States census, the city's population was 31,667, an increase of 948 from the 2010 census count of 30,719, which in turn reflected a decline of 621 from the 31,340 counted in the 2000 census. As of the 2020 census, it was the 6th-most-populous municipality in Monmouth County and had the 74th-highest population of any municipality in New Jersey.