Achanak (transl. Suddenly) is a 1973 Indian Hindi-language film, directed by Gulzar, written by Khwaja Ahmad Abbas, and starring Vinod Khanna. Gulzar received a Filmfare nomination as Best Director for this film. Even though Gulzar is an accomplished lyricist, this film did not have any songs in it. K. A. Abbas earned a Filmfare nomination for best story. It is inspired by the real-life sensational 1958 murder case K. M. Nanavati v State of Maharashtra.

The actual messages conveyed by the movie are more sublime than simple cuckoldry. First and foremost is the futility of a doctor's efforts to save life of an already condemned person. Tragically, the doctor saves the life of a person, who is already condemned. After months of untiring efforts of doctors, nurses and all paramedical staff, the condemned person finally meets his fate at the gallows. Ethics of human killing is another message. While on one hand the protagonist is awarded a Vir Chakra for killing several humans, on another he is punished to death for killing just two humans.

1963 movie Yeh Rastey Hain Pyar Ke was also based on the same case. Akshay Kumar starrer 2016 movie Rustom is based on the same case as well.

The film had no songs, the background score is composed by Vasant Desai.