Maxwell David Geismar (August 1, 1909 – July 1979) was an American writer, literary critic and biographer of Mark Twain.

Geismar wrote the introduction to Forgive My Grief: Volume II by Penn Jones Jr., which critiqued the Warren Commission. He also penned the introduction to Eldridge Cleaver's Soul on Ice (1968). A teacher at Sarah Lawrence College for many years, he signed "The Triple Revolution", sent to President Lyndon Johnson in 1964.