Karthik Sivakumar (born 25 May 1977), is an Indian actor who works predominantly in Tamil cinema. He has won three Filmfare Awards South, an Edison Award, a SIIMA Award and a Tamil Nadu State Film Award.

The younger brother of actor Suriya and son of actor Sivakumar, Karthi initially joined Mani Ratnam as an assistant director and made his acting debut with Paruthiveeran (2007), which earned him several accolades including the Filmfare Award for Best Actor. He went on to star in notable films like Aayirathil Oruvan (2010), Paiyaa (2010), Naan Mahaan Alla (2010), Siruthai (2011), Biriyani (2013), Madras (2014), Thozha (2016), Theeran Adhigaaram Ondru (2017), Kadaikutty Singam (2018), Kaithi (2019), Thambi (2019), Sulthan (2021), Viruman (2022), Ponniyin Selvan: I (2022) and Sardar (2022), thus establishing himself as a commercially successful leading actor of Tamil cinema.

Apart from his film career, Karthi has also been involved in social welfare activities, encouraging fans to do likewise through the "Makkal Nala Mandram", a social welfare club that he inaugurated. In 2011, he became a cause ambassador to promote awareness of lysosomal storage diseases. As of 2015, he is the treasurer of the Nadigar Sangam.