William Kissam Vanderbilt (December 12 1849 – July 22 1920) was a member of the prominent United States Vanderbilt family.

The second son of William Henry Vanderbilt, from whom he inherited $60 million, he was for a time active in the management of the family railroads, though not much after 1903. His sons William Kissam Vanderbilt II (1878-1944) and Harold Stirling Vanderbilt (1884-1970) were the last to be active in the railroads, the latter losing a proxy battle for the New York Central Railroad in the 1950s.

William K. Vanderbilt's first wife was Alva Erskine Smith (1853-1933), who he married in 1875. Born in 1853 to a slave-owning Alabama family, she was the mother of his children and was instrumental in forcing their daughter Consuelo (1877-1964) to marry the 9th Duke of Marlborough in 1895. Not long after this the Vanderbilts divorced, William K. later marrying Anne Harriman Rutherford Sands and Alva marrying Oliver Hazzard Perry Belmont.

After the death of his brother Cornelius Vanderbilt II in 1899 he was generally regarded as head of the Vanderbilt family.

Like other members of his wealthy family, he built magnificent Vanderbilt houses. His homes included Idle Hour (1900) on Long Island, New York and Marble House (1892), designed by Richard Morris Hunt who also designed his 660 Fifth Avenue mansion (1883), in Newport.

William Kissam Vanderbilt died in Paris, France in 1920. His remains were brought home and interred in the Vanderbilt family vault in the Moravian Cemetery at New Dorp on Staten Island, New York.