祖父悖论是一种时间旅行的悖论，科幻故事中常见的主题。最先由法国科幻小说作家赫内·巴赫札维勒（René Barjavel）在他1943年的小说《不小心的旅游者》（Le Voyageur Imprudent）中提出。情景如下：

The grandfather paradox is a proposed paradox of time travel which results in an inconsistency through changing the past. The paradox was described as early as 1931, and even then it was described as "the age-old argument of preventing your birth by killing your grandparents". Early science fiction stories dealing with the paradox are the short story Ancestral Voices by Nathaniel Schachner, published in 1933, and the 1943 book by René Barjavel Future Times Three. The paradox is described as follows: the time traveller goes back in time and kills his grandfather before his grandfather meets his grandmother. As a result, the time traveller is never born. But, if he was never born, then he is unable to travel through time and kill his grandfather, which means the traveller would be born after all, and so on.