I Know What You Did Last Summer is a 1997 American slasher film directed by Jim Gillespie and written by Kevin Williamson. It stars Jennifer Love Hewitt, Sarah Michelle Gellar, Ryan Phillippe, and Freddie Prinze Jr., with supporting roles played by Johnny Galecki, Bridgette Wilson, Anne Heche, and Muse Watson. The first installment in the I Know What You Did Last Summer franchise, it is loosely based on the 1973 novel by Lois Duncan. The film centers on four teenage friends, who are stalked by a hook-wielding killer one year after covering up a car accident in which they supposedly killed a man. It also draws inspiration from the urban legend known as "the Hook", as well as the slasher films Prom Night (1980) and The House on Sorority Row (1982).

Williamson was approached to adapt Duncan's source novel by producer Erik Feig. Where Scream, released the previous year, contained prominent elements of satire and self-referentiality, Williamson's script for I Know What You Did Last Summer reworked the novel's central plot to resemble a straightforward 1980s-era slasher film.

I Know What You Did Last Summer was released theatrically in the United States on October 17, 1997. It received a mixed reception from critics and was a sizeable commercial hit, grossing $125.3 million worldwide on a budget of $17 million, staying in first place at the U.S. box office for three consecutive weeks. The film was parodied in Scary Movie (2000) and is frequently referenced in popular culture, as well as being credited alongside Scream with revitalizing the slasher genre in the 1990s.

The film was followed by a sequel, I Still Know What You Did Last Summer (1998), in which Hewitt, Prinze Jr., and Watson reprised their roles. A straight-to-video standalone sequel, I'll Always Know What You Did Last Summer (2006), featured an entirely new cast. Following a television series adaptation released by Amazon Prime Video in 2021, a direct legacy sequel to the first two films is scheduled to be released by Sony Pictures on July 18, 2025.