S. Alan Stern (born 22 November 1957, New Orleans, Louisiana) is an American planetary scientist. He is the principal investigator of the New Horizons mission to Pluto and has over 25 years of space science experience, including 200 technical papers, 40 popular articles and has authored/co-authored two books, and served as editor on three technical volumes, and three collections of scientific essays.

Stern has been involved as a researcher in 24 suborbital, orbital, and planetary space missions, including eight for which he was the mission principal investigator including the Southwest Ultraviolet Imaging System, an instrument which flew on two space shuttle missions, STS-85 in 1997 and STS-93 in 1999.

Stern has also led the development of eight scientific instruments for planetary and near-space research missions and has been a guest observer on numerous NASA satellite observatories, including the International Ultraviolet Explorer, the Hubble Space Telescope, the International Infrared Observer and the Extreme Ultraviolet Observer and has given over 100 hundred technical talks and dozens of popular lectures and speeches about astronomy and the space program. Stern was Executive Director of the Southwest Research Institute's Space Science and Engineering Division until becoming Associate Administrator of NASA's Science Mission Directorate in 2007. He resigned from that position after nearly a year. In early 2009 Dr. Stern's name has been mentioned as a potential contender for the position of NASA administrator under President Obama's Administration. Stern has stated however that he is not interested in the position at this time given his desire to spend time with his family.