Yoshio Masui (増井 禎夫, Masui Yoshio, born January 1, 1931) is a Japanese-Canadian cell biologist.^[1] Masui retired in 1997 and has since held the position of Professor Emeritus at the University of Toronto.^[2][3]

Masui studied biology at Kyoto University, graduating with his Bachelor of Science degree in zoology in 1953, his Master of Science in 1955 and his Ph.D. in 1961.^{[citation needed]}

While still studying at Kyoto University, he taught biology, first as a teacher's assistant and then as a teacher, at Konan University, where he was promoted to assistant professor after his earning his Ph.D. In 1966, he moved to Yale University to join Clement L. Markert's lab, and in 1969 to the University of Toronto, where he taught as associate professor in the Department of Zoology.^{[citation needed]}

In 1998, he won the Albert Lasker Award for Basic Medical Research with Lee Hartwell and Paul Nurse for their pioneering work on cell division.^[4] He was elected a Fellow of the Royal Society (FRS) in 1998^[5] and an officer of the Order of Canada in 2003 in recognition of his life's work. in 1992 he was awarded the Gairdner Foundation International Award.