Karl Frithiof Sundman (1873 – 1949) was a Finnish mathematician who used analytic methods to prove the existence of a convergent infinite series solution to the three-body problem in 1906 and 1909.

He was awarded the Pontécoulant prize by the French Academy of Science in 1913 for this work. He also published a paper on regularization methods in mechanics in 1912.

In 1947, Sundman was elected a foreign member of the Royal Swedish Academy of Sciences.

The crater Sundman on the Moon is named after him, as is the asteroid 1424 Sundmania.