William Morrow Knox Olcott (August 27, 1862 – May 10, 1933) was an American lawyer and politician from New York City.
He was a member of the Board of Aldermen from January 1895 until his appointment as New York County District Attorney He was appointed New York County District Attorney in December 1896 to fill the vacancy caused by the death of John R. Fellows.
In November 1897, he ran on the Republican ticket to succeed himself, but was defeated by Democrat Asa Bird Gardiner.
In December 1897, Olcott was appointed by Governor Frank S. Black, a judge of the New York City Court, to fill the vacancy caused by the resignation of Robert Anderson Van Wyck who had been elected Mayor.
On December 6, 1888, he married Jessica Augusta Baldwin, and their son was Nellson Olcott, who became an Assistant District Attorney under Edward Swann and Joab H. Banton.