Yrjö Väisälä (September 6, 1891 - July 21, 1971) was a Finnish astronomer and physicist.
[A note on dates: the birth date is most likely a Gregorian calendar date, because although Finland was part of Russia at the time and Russia used the Julian calendar until 1918, the Gregorian calendar continued in official use in Finland because Finland had been part of Sweden when Sweden had converted to the Gregorian calendar in 1753.]
His main contributions were in the field of optics. He developed several methods for measuring the quality of optical elements. This allowed the construction of some of the earliest high-quality Schmidt telescopes. Some historians even claim that Väisälä invented the Schmidt telescope independently, but this has remained an open question. The Schmidt telescopes he built were used at the University of Turku for searches of asteroids and comets. His research group discovered several comets and more than 800 asteroids.
Notably discovered the periodic comets 40P/Vaisala and 139P/Vaisala-Oterma (the latter co-discovered with Liisi Oterma).
A crater on the Moon is named after him, and so is the asteroid 1573 Väisälä .