Anniyan (/ənnɪjən/ transl. Stranger) is a 2005 Indian Tamil-language psychological action thriller film directed by Shankar and produced by V. Ravichandran. The film stars Vikram as a meek law-abiding lawyer who suffers from dissociative identity disorder, and develops two other identities: a playboy fashion model, and a murderous vigilante hellbent on a mission of eradicating corruption. The cast also includes Sadha, Prakash Raj, Vivek, Nedumudi Venu, and Nassar.

Shankar conceived the film in mid-2003 during the post-production period of his previous film Boys. He based the film on his own life experiences during his formative years when he was disturbed by what he saw around him and his eventual displeasure with the society. Pre-production for Anniyan began in November 2003 and principal photography in March 2004. The making of the film, which included numerous production delays, took 14 months. The film was shot at Hyderabad, Thanjavur, Viluppuram and Chennai. The song sequences were filmed in Mumbai, Malaysia, Amsterdam and Tenkasi. The film was notable for its recreation of the Tyagaraja Aradhana music festival and the extensive use of time slice photography in an action sequence.

Cinematographer V. Manikandan discontinued the project halfway through, until he was replaced with Ravi Varman. The technical departments were headed by V. T. Vijayan (editing), Sabu Cyril (production design), and Peter Hein (action choreography). The soundtrack was composed by Harris Jayaraj, who was in his first collaboration with the director. The film was touted as the director's magnum opus and was budgeted at ₹26.3 crore, making it the most expensive Indian film during the time of its release. Notably, it was the first South Indian film to obtain institutional finance, and it had the highest insurance coverage available for films at that time.

Anniyan was released on 17 June 2005 and became a commercial success, and in addition to winning a record breaking eight Filmfare Awards and six State Film Awards, it also won a National Award in the Special Effects category.

Ramanujam "Ambi" Parthsarthy, a straightforward consumer protection advocate from Thiruvallikeni, Chennai, expects everyone to follow the law and attempts to prosecute those who violate it, but his efforts are unsuccessful due to circumstantial evidence favouring the accused. His efforts to raise civic awareness go unacknowledged due to general lack of seriousness and widespread corruption. Ambi's suppressed anger manifests as an alter ego called Anniyan, a grim reaper-themed vigilante, who seeks out corrupt and indifferent individuals to eliminate them. Anniyan creates a website, where he compiles a list of wrongdoers and kills them, using punishments described in the ancient Hindu scripture Garuda Purana.

Ambi secretly loves his neighbor Nandini, an aspiring Carnatic singer, but never expresses his feelings due to fear of rejection. When he gets the courage to propose to her during the annual Tyagaraja Aradhana with the help of his friend SI Chari, she rejects him as she cannot bear his overbearing nature, constant complaining, and nitpicking. Distraught, Ambi attempts suicide, almost drowning himself before having second thoughts. He subsequently develops another personality named Remo, a metrosexual fashion model. Nandini becomes smitten with Remo and falls in love with him, and their marriage is eventually fixed.

While purchasing a plot of land for her dowry, Nandini undervalues the property to evade stamp duty. Ambi, who accompanies her to the government office, refuses to help her. Later, when Nandini and Remo are on a date, Remo transforms into Anniyan and attempts to punish her for corruption. As Anniyan is about to kill her, Nandini calls out for Ambi and Anniyan reverts to Ambi, who collapses and loses consciousness. Nandini takes Ambi to NIMHANS, where he is diagnosed with multiple personality disorder. Through recovered-memory therapy, Vijaykumar, the chief psychiatrist, uncovers Ambi's past. When Ambi was ten years old, he witnessed the accidental death of his younger sister Vidya due to civic apathy. The incident left a deep emotional scar, which was the reason for his lofty ideals. Vijaykumar realises that although Anniyan and Remo know about Ambi, albeit as a separate person, Ambi is oblivious to their existence within him. Vijaykumar declares that Remo will cease to exist if Nandini reciprocates Ambi's feelings, but Anniyan will cease to exist only when society reforms. Nandini accepts Ambi's love and Remo disappears.

Meanwhile, DCP Prabhakar and Chari investigate the murders committed by Anniyan. In disguises, they discover clues left behind by Anniyan, which are the names of the punishments meted out to victims. Prabhakar is personally determined to punish Anniyan as one of Anniyan's victims, Chockalingam, an errant catering contractor with the Indian Railways, was his elder brother. In a dramatic publicity stunt, Anniyan admits to the murders when he appears amidst the public and the press. Anniyan explains the rationale behind the murders and adds that only when every Indian is responsible and sincere can the country prosper on par with developed nations. His methods draw both praise and criticism. Prabhakar tries to catch Anniyan, but he escapes.