Marshall Kay (November 10, 1904 – September 4, 1975) was a geologist and professor at Columbia University.
He is best known for his studies of the Ordovician of New York, Newfoundland, and Nevada, but his studies were global and he published widely on the stratigraphy of the middle and upper Ordovician.
Kay's careful fieldwork provided much geological evidence for the theory of continental drift.
Less well known is his work for the Manhattan Project, as a geologist searching for manganese deposits.
His son Richard Kay of Duke University is a biological anthropologist and vertebrate paleontologist.