William Thaw Sr. (October 12, 1818 – August 17, 1889) was an American businessman who made his fortune in transportation and banking.

He underwrote the building of the Allegheny Observatory for John Brashear, considered at the time one of the ten best in the world.

Lyndhurst, completed in 1889 and demolished around 1942, was located at 1165 Beechwood Boulevard in the Squirrel Hill neighborhood of Pittsburgh.

[14] He was the father of Harry Kendall Thaw, whose 1906 killing of noted architect Stanford White resulted in a sensational "trial of the century" and aftermath.

A small town in East Central Illinois is named Thawville, Illinois in honor of William Thaw Sr. William K. Thaw owned a controlling interest in the Gilman, Clinton & Springfield Railroad and Thawville was platted in November 1871.