Vilhelm Friman Koren Bjerknes (March 14, 1862 - April 9, 1951) was a Norwegian physicist and meteorologist who did much to found the modern practice of weather forecasting.

Born in Christiania, Bjerknes enjoyed an early exposure to fluid dynamics, assisting his father, Carl Anton Bjerknes , in his mathematical research. In 1890, he became assistant to Heinrich Hertz and made substantial contributions to Hertz' work on electromagnetic resonance.

In 1895, he became professor of applied mechanics and mathematical physics at the University of Stockholm where he elucidated the fundamental interaction between fluid dynamics and thermodynamics. It was this work that inspired both V. Walfrid Ekman and Carl-Gustav Arvid Rossby to apply it to large-scale motions in the oceans and atmosphere and to make modern weather forecasting feasible. Bjerknes himself had forseen the possible applications as early as 1904.

In 1907, Bjerknes returned to the University of Kristiania before becoming professor of geophysics at the University of Leipzig in 1912. In 1917, he founded the Bergen Geophysical Institute where he wrote his book On the Dynamics of the Circular Vortex with Applications to the Atmosphere and to Atmospheric Vortex and Wave Motion (1921). From 1926 to his retirement in 1932 he held a position at the University of Oslo, where he died.

Craters on Mars and the Moon were named in his honor.