What is now Belmar was originally incorporated as Ocean Beach borough by an Act of the New Jersey Legislature on April 9, 1885, from portions of Wall Township, based on the results of a referendum held two days earlier. On April 16, 1889, it became the City of Elcho borough, which lasted for a few weeks until the name was changed to the City of Belmar Borough as of May 14, 1889. The city acquired its current name, Borough of Belmar, on November 20, 1890.
As of the census of 2000, there were 6,045 people, 2,946 households, and 1,316 families residing in the borough. The population density was 5,921.7 people per square mile (2,288.2/km²). There were 3,996 housing units at an average density of 3,914.5/sq mi (1,512.6/km²). The racial makeup of the borough was 91.53% White, 3.46% Black, 0.18% Native American, 1.03% Asian, 1.99% from other races, and 1.82% from two or more races. Hispanic or Latino of any race were 6.85% of the population.
There were 2,946 households out of which 17.2% had children under the age of 18 living with them, 32.0% were married couples living together, 9.0% had a female householder with no husband present, and 55.3% were non-families. 44.3% of all households were made up of individuals and 12.1% had someone living alone who was 65 years of age or older. The average household size was 2.05 and the average family size was 2.92.
In the borough the population was spread out with 17.2% under the age of 18, 7.6% from 18 to 24, 36.7% from 25 to 44, 22.7% from 45 to 64, and 15.7% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 99.8 males. For every 100 females age 18 and over, there were 99.2 males.
The median income for a household in the borough was $44,896, and the median income for a family was $61,250. Males had a median income of $40,557 versus $34,323 for females. The per capita income for the borough was $29,456. About 4.5% of families and 8.6% of the population were below the poverty line, including 11.3% of those under age 18 and 7.9% of those age 65 or over.
As of the 2000 census, 28.2% of Belmar's residents were of Irish ancestry, the 12th highest of any municipality in New Jersey, for all communities in which at least 1,000 people listed their ancestry.