Moodu Pani (English: The Mist) is a 1980 Indian Tamil-language psychological thriller film written, directed and filmed by Balu Mahendra. Starring his then wife Shoba and Pratap, with N. Viswanathan, Gandhimathi, Mohan and Bhanu Chander in supporting roles, it is based on two novels: Idhuvum Oru Viduthalai Thaan (1978) by Rajendra Kumar, and The Collector (1963) by John Fowles. The film follows Chandru (Pratap), who has a strong hatred towards prostitutes and would kill any such woman he encounters. He falls in love with Rekha (Shoba) and believes marrying her will end his psychological distress.

Moodu Pani was the third directorial venture of Mahendra and his second in Tamil after Azhiyatha Kolangal (1979). It also marked Mohan's debut in Tamil cinema, and the last film Shoba acted in before her death. Principal photography took place between January and April 1980, mostly in Udupi, Bangalore and Ooty. The film's similarities to Alfred Hitchcock's film Psycho (1960) have been widely discussed. The soundtrack was composed by Ilaiyaraaja, this being his 100th film, and his brother Gangai Amaran was the main lyricist.

Moodu Pani was released on 6 November 1980, during the Diwali holiday frame. The film received critical acclaim, with particular praise for the performances of Pratap and Shoba, and Ilaiyaraaja's music; the song "Yen Iniya Pon Nilavae" attained immense popularity. It was a commercial success, running for over 200 days in theatres, and responsible for a resurgence of the thriller genre in Tamil cinema at that point. Despite the film propelling Pratap to stardom, it also led to him being typecast in similar roles.