Magnus Gustaf (Gösta) Mittag-Leffler (16 March 1846 – 7 July 1927) was a Swedish mathematician.

Mittag-Leffler was born in Stockholm, son of the school principal John Olof Leffler and Gustava Wilhelmina Mittag; he later added his mother's maiden name to his paternal surname. His sister was the writer Anne Charlotte Leffler. He matriculated at Uppsala University in 1865, completed his Ph.D. in 1872 and became docent at the university the same year. He was also curator (chairman) of the Stockholm Nation (1872 – 1873). He next traveled to Paris, Göttingen and Berlin before taking up a position as professor of mathematics at the University of Helsinki 1877 – 1881 and then as the first professor of mathematics at the University College of Stockholm (the later Stockholm University); he was president of the college 1891 - 1892 and retired from his chair in 1911. Mittag-Leffler went into business and became a successful businessman in his own right, but an economic collapse in Europe wiped out his fortune in 1922.

He was a member of the Royal Swedish Academy of Sciences (1883), the Finnish Society of Sciences and Letters (1878, later honorary member), the Royal Swedish Society of Sciences in Uppsala, the Royal Physiographic Society in Lund (1906) and about 30 foreign learned societies, including the Royal Society of London (1896) and Académie des sciences in Paris. He held honorary doctorates from the University of Oxford and several other universities.

Mittag-Leffler was a convinced advocate of women's rights and was instrumental in making Sofia Kovalevskaya a full professor of mathematics in Stockholm - the first woman anywhere in the world to hold that position.

Mittag-Leffler founded the mathematical journal Acta Mathematica (1882), partly paid for with the fortune of his wife Signe Lindfors, who came from a very wealthy Finnish family, and collected a large mathematical library in his villa in the Stockholm suburb of Djursholm. The house and its contents was donated to the Academy of Sciences as the Mittag-Leffler Institute.

A legend that Alfred Nobel did not set up a prize in Mathematics because of a thwarted affair with Signe Lindfors is not supported by historical evidence.