Karthik Sivakumar (born 25 May 1977), better known by his stage name Karthi, is an Indian film actor who works primarily in the Tamil film industry. The younger brother of actor Suriya, Karthi holds a Bachelor's degree in Mechanical Engineering from India and a Master of Science in Industrial Engineering which he earned in the United States. He however always wanted to become a film director, and hence returned to India and joined Mani Ratnam as an assistant director. He was offered acting roles and made his acting debut in Paruthiveeran in 2007 as the title character, a careless village ruffian, winning critical acclaim and several accolades including the Film fare Award for Best Actor and a Tamil Nadu State Film Award. His next role was that of a coolie in Aayirathil Oruvan (2010), an adventure fantasy film directed by Selvaraghavan. He achieved consecutive commercial successes with his subsequent releases – Paiyaa (2010), Naan Mahaan Alla (2010) and Siruthai (2011). After appearing in a series of box office flops, he starred in successful films such as Biriyani (2013), Madras (2014) Oopiri (2016), Kaatru Veliyidai (2017) and Theeran Adhigaaram Ondru (2017), which established him in 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 disease. As of 2015, he is the treasurer of the Nadigar Sangam.