A sleeper hit, also known as a surprise hit, sleep-killer or sleeper, refers to a film, book, single, album, TV show, or video game that gains unexpected success or recognition. Sleeper hits often grow in popularity over time as people promote them through word-of-mouth marketing.

Some sleeper hits achieve unexpected success at the box office immediately upon their initial theatrical release, but this is not typical. Because these films are not expected to do particularly well they often receive little promotion or advertising and take time to register with the public. Typically the sleeper hit relies instead on positive "word of mouth" as well as the publicity generated by awards and good reviews. Two good examples of these are Mike Judge's Office Space and Idiocracy, both of which quickly became cult classics. The movie Caddyshack is another good example.

Thus sleeper films often attract the most viewers in the latter part of their theatrical releases. There's Something About Mary had a small release in 1998, but gained notoriety through word of mouth, and grew to top the weekend box office on its 8th week of release. Similarly, the 2010 film How to Train Your Dragon was considered a relative disappointment when its premiere wide release box office gross was smaller than expected, but was later reassessed as a major success when its theatrical run proved unexpectedly strong over a longer term to more than compensate. Studios have become more adept at promoting sleeper success at the box office, gradually increasing the number of screens and amount of advertising devoted to a promising film over several weeks (as opposed to a traditional pre-release advertising blitz). Some sleeper hits fail completely at the box office but succeed later on television and video as cult films (such as The Shawshank Redemption and The Big Lebowski) through positive word-of-mouth reviews communicated between fans in particular social networks or demographics.