Si3 (short for Singam 3, transl. Lion 3) is a 2017 Indian Tamil-language action film directed by Hari and jointly produced by Studio Green along with Aadnah Arts. The film stars Suriya reprising his role as S. Duraisingam, alongside Anushka Shetty, Shruti Haasan, Thakur Anoop Singh, Robo Shankar, Soori, Krish, Radhika Sarathkumar, Sarath Babu and Vijayakumar. It is the third iinstalment in the Singam film series and is the sequel to Singam II (2013). The film follows Duraisingam, the deputy commissioner in Visakhapatnam, who battles against a criminal operation dealing in deadly toxic waste.

The film was officially announced in October 2014 under the initial official title S3, the short-term for Singam 3; however, it was later changed to Si3 to exploit the Tamil Nadu Government's rule of entertainment tax exemption for films titled in Tamil. Principal photography commenced in January 2016. It was shot for the most part in Visakhapatnam. Devi Sri Prasad who is supposed to compose the film's music was replaced by Harris Jayaraj, cinematography was handled by Priyan and editing by V. T. Vijayan.

Si3 was released on 9 February 2017 to mixed reviews from critics and grossed ₹110 crore at the box office.