'Luís Ferdinand Cruls' (21 January 1848 – 21 June 1908) was a Belgian astronomer who did most of his work in Brazil. He is also known as Louis Cruls.
Cruls was born in Diest, Belgium, and died in Paris, France. He served in the Belgian Army from 1869 to 1872. In 1881, he accepted the position of director of the Observatory of Rio de Janeiro.
In 1878, he published his work on the planet Mars. He observed the transit of Venus in 1882 from Punta Arenas, and was one of the astronomers (along with William Henry Finlay) credited with the discovery of the Great Comet of 1882, which went on to become perhaps the brightest comet in recorded history.
A crater on Mars was named in his honor.