Leo Reino Sario (18 May 1916, Lieksa – 15 August 2009, Santa Monica, California) was a Finnish-born mathematician who worked on complex analysis and Riemann surfaces.
After service as a Finnish artillery officer in the Winter War and WWII, he received his PhD in 1948 under Rolf Nevanlinna at the University of Helsinki. Nevanlinna and Sario were founding members of the Academy of Finland, and there is a statue on the Academy grounds named after Sario. Sario moved to the United States in 1950 and obtained temporary positions at the Institute for Advanced Study, MIT, Stanford University, and Harvard University. In 1954 he became a professor at UCLA, remaining there until his retirement in 1986. He was the author or co-author of five major books on complex analysis and over 130 papers. He supervised 36 doctoral students, including Kōtarō Oikawa and Burton Rodin. In 1957 he was awarded the Cross of the Commander of Finland's Order of Knighthood.