John Mace Grunsfeld is an American physicist and a former NASA astronaut. He is a veteran of five Space Shuttle flights and has served as NASA Chief Scientist. His academic background includes research in high energy astrophysics, cosmic ray physics and the emerging field of exoplanet studies with specific interest in future astronomical instrumentation. After retiring from NASA in 2009, he served as the deputy director of the Space Telescope Science Institute in Baltimore, Maryland. In January 2012, he returned to NASA and served as associate administrator of NASA's Science Mission Directorate (SMD). Grunsfeld announced his retirement from NASA in April 2016.

Chief Scientist is the most senior science position at the National Aeronautics and Space Administration (NASA). The chief scientist serves as the principal advisor to the NASA Administrator in science issues and as interface to the national and international science community, ensuring that NASA research programs are scientifically and technologically well founded and are appropriate for their intended applications.