Margaret B. Lettvin is an American writer and promoter of exercise and health. She was known in the Boston area in the 1970s for a PBS television show on WGBH-TV called Maggie and The Beautiful Machine, and a book based on the show. After a serious car accident, she developed a set of exercises for back pain, which became Maggie's Back Book ().
She is the widow of MIT professor Jerry Lettvin, with whom she served as houseparent of the MIT Bexley dorm.