John Mace Grunsfeld (Ph.D.) (born October 10, 1958) is an American physicist and a NASA astronaut. He is a veteran of five spaceflights and has also served as NASA Chief Scientist.

Born in Chicago, Illinois. Married to the former Carol E. Schiff. They have two children. John enjoys mountaineering, flying, sailing, bicycling, and music. His father, Ernest A. Grunsfeld III, resides in Highland Park, Illinois.

Graduated from Highland Park High School, Highland Park, Illinois, in 1976; received a bachelor of science degree in physics from the Massachusetts Institute of Technology in 1980; a master of science degree and a doctor of philosophy degree in physics from the University of Chicago in 1984 and 1988, respectively.