Duncan McMartin Jr. (1776 – October 3, 1837) was an American politician from New York.
He lived in Broadalbin, then in Montgomery Co., now in Fulton County.
He was a member of the New York State Assembly (Hamilton & Montgomery Co.) in 1819.
He was a member of the New York State Senate (Eastern D.) from 1820 to 1822, sitting in the 43rd, 44th and 45th New York State Legislatures.
He was buried at a cemetery now located in Perth, a town formed after McMartin's death, and which includes an area formerly belonging to Broadalbin.