S. K. Senthivel is a Sri Lankan political activist. He has been guiding the New-Democratic Marxist-Leninist Party as the General Secretary since 1989.^[1][2][3]

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Sinnathamby Kasipillai Senthivel was born in Sirrupiddy near Puttur, Sri Lanka. He studied at Sri Somaskanda College.^[4]

He was inspired by public opinion at an early age and began to act as a Marxist. At the age of 21 he became full time worker for Ceylon Communist Party (Maoist) in 1964 and participated in the 1966 October 21 Uprising against Untouchability in Jaffna.^[5] He subsequently took part in a number of mass struggles and written many books in Tamil.^[6]

In 1988, he co-authored Caste and its struggles in Sri Lanka with Dr. Nadesan Raveendran.^[5] In 1994, he wrote a book on sociology, human beings and social life. Subsequently in 2003, he wrote a biography of Communist M. Karthikeyan and the Northern Left Movement. Following in 2014, he wrote about K. A. Subramaniam the founder of the New-Democratic Marxist-Leninist Party on his 25th anniversary.^[6]