William Stephen Finsen (July 28, 1905 – May 16, 1979) was a South African astronomer.
He discovered a number of double stars and took many photographs of Mars. He developed the Finsen eyepiece interferometer to measure very close double stars. He was the final director of Union Observatory in South Africa from 1957 until it closed in 1965 (it was renamed Republic Observatory in 1961).
The asteroid 1794 Finsen is named after him, as is the geological feature Finsen Dorsum on the asteroid 433 Eros.