James Greenleaf (June 9, 1765 – September 17, 1843) is an important early American land speculator.

A member of a prominent and wealthy Boston family, he marriea a Dutch noblewoman (whom he later abandons, then divorces) and is briefly consul at the United States embassy in Amsterdam.

Returning to the United States, he engaged in land speculation in the District of Columbia, New York state, and other areas.

He is a central figure in the early development of Washington, D.C.

His land business collapses in 1797, and he spends a year in debtor's prison.

He marries a wealthy Pennsylvania heiress after his release, and spends the remainder of his life in genteel poverty, fending off lawsuits.