Hainesburg is an unincorporated community and census-designated place (CDP)[9] located within Knowlton Township in Warren County, in the U.S. state of New Jersey.

[10] While the community has existed for over a century, the CDP was designated as part of the 2010 United States Census.

[14] Hainesburg is located next to Paulins Kill, and a stone grist mill was erected on that waterway prior to the American Revolution.

Originally known as "Sodom", the town's first post office was called "Hainesburg" in honor of John Haines, who made a significant donation to a school located there.

[15][16] A saw mill was built by George Adams in 1881, and a station on the New York, Susquehanna and Western Railway was located in Hainesburg.