Was New Jersey a slave state?

Yes and no. New Jersey was one of the original Thirteen Colonies, and like most of them, it allowed slavery. However, over the course of the 18th century, New Jersey gradually passed laws that restricted and regulated slavery and it was considered to be a "middle-ground" state on the issue.

In the lead-up to the Revolutionary War, New Jersey, like several of the other northern states, began taking steps to abolish slavery—in part to undermine the social and economic power of British loyalists, the majority of whom were slaveholders. At the end of the American Revolutionary War, New Jersey passed the Gradual Manumission Act of 1786, which stated that all children born to enslaved women after July 4, 1798 would be free. It also set up a system to allow current enslaved people to buy their freedom or to sue their owners for mistreatment.

Despite the Gradual Manumission Act, New Jersey continued to allow slavery until well after the ratification of the United States Constitution in 1788. Finally, in 1804, New Jersey became one of the last of the original Thirteen Colonies to abolish slavery.