Macomb's Purchase is a large historical area of northern New York, USA purchased from the state in 1791 by Alexander Macomb, who had become rich as a merchant in the American Revolution.
It was 3,670,715 acres (14,855 km²). The tract included much of northern New York, along the St. Lawrence River and eastern Lake Ontario, including the Thousand Islands, at about eight cents an acre. The purchase was divided into ten large townships. From this purchase are derived the deeds for all the lands that are now included in Lewis, Jefferson, St. Lawrence and Franklin Counties, as well as portions of Herkimer and Oswego Counties.
The Macomb Purchase was especially successful in straightening out the previously tangled Mahair Purchase of 1786.