Willem de Sitter (6 May 1872 – 20 November 1934) was a Dutch mathematician, physicist, and astronomer.

De Sitter made major contributions to the field of physical cosmology.

He co-authored a paper with Albert Einstein in 1932 in which they discussed the implications of cosmological data for the curvature of the universe.

De Sitter was also well-known for his research on the motions of the moons of Jupiter, invited to give the George Darwin Lecture at the Royal Astronomical Society in 1931.

Another son of Willem, Aernout de Sitter (1905 – 15 September 1944[7]), was the director of the Bosscha Observatory in Lembang, Indonesia (then the Dutch East Indies), where he studied the Messier 4 globular cluster.