Bloomsbury is a borough in Hunterdon County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 870, reflecting a decline of 16 (-1.8%) from the 886 counted in the 2000 Census, which had in turn declined by 4 (-0.4%) from the 890 counted in the 1990 Census.
Bloomsbury was incorporated as a borough by an act of the New Jersey Legislature on March 30, 1905, from portions of Bethlehem Township.
The Borough of Bloomsbury was once known as "Johnson's Iron Works", owned by Robert Johnson, on the north bank of the river. The current name is derived either from the Bloom family, influential in the early history of the town, or from the iron ore processed into masses of wrought iron that are known as "blooms".
According to the United States Census Bureau, the borough had a total area of 0.96 square miles (2.49 km2), including 0.93 square miles (2.41 km2) of land and 0.03 square miles (0.08 km2) of water (3.23%).
The borough borders the municipalities of Bethlehem Township in Hunterdon County; and Franklin Township, Greenwich Township and Pohatcong Township in Warren County. Bloomsbury is located on the south side of the Musconetcong River in the northwest corner of Hunterdon County and is the third-smallest municipality in Hunterdon County.
The 2010 United States Census counted 870 people, 337 households, and 237 families in the borough. The population density was 991.9 inhabitants per square mile (383.0/km2). There were 358 housing units at an average density of 408.1 per square mile (157.6/km2). The racial makeup was 95.40% (830) White, 1.03% (9) Black or African American, 0.00% (0) Native American, 1.84% (16) Asian, 0.00% (0) Pacific Islander, 0.34% (3) from other races, and 1.38% (12) from two or more races. Hispanic or Latino of any race were 4.02% (35) of the population.
Of the 337 households, 38.6% had children under the age of 18; 57.3% were married couples living together; 8.3% had a female householder with no husband present and 29.7% were non-families. Of all households, 24.0% were made up of individuals and 6.8% had someone living alone who was 65 years of age or older. The average household size was 2.58 and the average family size was 3.09.
27.0% of the population were under the age of 18, 5.3% from 18 to 24, 26.9% from 25 to 44, 31.6% from 45 to 64, and 9.2% who were 65 years of age or older.