Sir Thomas Muir (25 August 1844-21 March 1934) was a Scottish mathematician, remembered as an authority on determinants. He was born in Stonebyres in Lanarkshire, and brought up in Biggar. At the University of Glasgow he changed his studies from classics to mathematics after advice from the future Lord Kelvin. After graduating he held positions at St. Andrews University and Glasgow University. From 1874 to 1892 he taught at Glasgow High School . In 1882 he published Treatise on the theory of determinants; then in 1890 he published a History of determinants.

From 1892 he was in South Africa working in education, and then in administration at the University of the Cape . He was knighted in 1910.

From 1906 onwards he published a five-volume expansion of his history of determinants, the final part (1929) taking the theory to 1920. A further book followed in 1930.

His name now attaches to a duality theorem on relations between minors. In more abstract language, it is a general result on the equations defining Grassmannians as algebraic varieties.