William Sadler Franks (26 April 1851 – 19 June 1935) was a British astronomer.
He published a catalogue of the colors of 3890 stars.
In 1910 he was hired by F. J. Hanbury (of the Allan and Hanbury firm) to work as an observer at Hanbury's private Brockhurst Observatory at East Grinstead in Sussex. He continued his studies of the colors of stars there, and also made micrometer measurements of double stars. In 1923 he won the Jackson-Gwilt Medal of the Royal Astronomical Society.
One of the other observers who used the Brockhurst Observatory with Franks was a teenage Patrick Moore (fourteen years old at Franks's death).