Mary Belle Allen (November 11, 1922, Morristown, New Jersey –1973, Fairbanks, Alaska) was an American botanist, chemist, mycologist, algologist, and plant pathologist, and a pioneer of biochemical microbiology. With Daniel I. Arnon and F. Robert Whatley, she did breakthrough research discovering and demonstrating the role of chloroplasts in photosynthesis. In 1962 she received the Darbaker Prize from the Botanical Society of America for her work on microbial algae. In 1967 she was nominated jointly (but unsuccessfully) with Arnon and Whatley for a Nobel Prize.