Harold S. "Hal" Johnston (October 11, 1920 – October 20, 2012) was an American scientist who studied chemical kinetics and atmospheric chemistry. After beginning his teaching career at Stanford University, he was a faculty member and administrator at the University of California, Berkeley for nearly 35 years. In 1971, Johnston authored a paper suggesting that environmental pollutants could erode the ozone layer.
Johnston was elected to several scholarly organizations, including the National Academy of Sciences, the American Academy of Arts and Sciences and the American Association for the Advancement of Science. He won the National Medal of Science in 1997.