Constance Elaine Trimmer Willis (born December 31, 1945) is an American science fiction writer. She is one of the most honored science fiction writers of the 1980s and 1990s.

She has won, among other awards, eight Hugo Awards and six Nebula Awards. She lives in Greeley, Colorado with her husband, a professor of physics at the University of Northern Colorado. She also has one daughter, Cordelia.

Willis is known for her accessible prose and likable characters. She has written several pieces involving time travel and a group of historians based at Oxford in the future. These pieces include her Hugo Award-winning novels Doomsday Book and To Say Nothing of the Dog and the short story «Fire Watch,» found in the short story collection of the same name.