J. S. Foster was a Canadian physicist, born in Nova Scotia. He was educated at Yale University and became an assistant professor at McGill University in 1924, where he taught physics. He became associate professor in 1930. He was made a Fellow of the Royal Society of Canada in 1929 and the Royal Society of London in 1935. During the Second World War, he served as a liaison officer for the National Research Council, working at the MIT-run Radiation Laboratory to create a radar antenna now known as the "Foster scanner." He returned to McGill after the war. In 1949, a cyclotron was commissioned at McGill, which Foster oversaw. He served as chairman of the McGill Physics Department from 1952 to 1954.

He returned to McGill in 1944, where he directed the construction of a 100-MeV cyclotron. This instrument was commissioned in 1949. At the time this was the second largest in the world. From 1952 until 1954 he was chairman of the physics department at McGill. He died in Berkeley, California. The John Stuart Foster Radiation Laboratory and Cyclotron at McGill was named after him in 1964.