Emmanuel Liais (15 February 1826 – 5 March 1900) was a French-Brazilian astronomer, botanist and explorer who spent many years in Brazil.

He became a close acquaintance of the Brazilian Emperor Dom Pedro II, and became the director of the Imperial Observatory at Rio de Janeiro from January to July 1871 and again from 1874 to 1881.

He made astronomical observations of Mars and in 1865 speculated that the dark albedo features were vegetation and not water (in fact, as we know today, they are neither).

At the behest of the emperor, he made extensive exploration expeditions within Brazil and studied the plants of remote regions, sending some of them to France.

He wrote a book entitled Climats, géologie, faune et géographie botanique du Brésil (Paris: Garnier Frères, 1872).