The Lanka Sama Samaja Party, often abbreviated as LSSP, is a major Trotskyist political party in Sri Lanka. It was the first political party in Sri Lanka, having been founded in 1935 by Leslie Goonewardene, N.M. Perera, Colvin R. de Silva, Philip Gunawardena and Robert Gunawardena. It currently is a member of the main ruling coalition in the government of Sri Lanka and is headed by Tissa Vitharana. The party was founded with Leninist ideals, and is classified as a party with socialist aims.

Leslie Simon Goonewardene was a prominent Sri Lankan statesman. He founded Sri Lanka's first political party, the Lanka Sama Samaja Party, in 1935, and served as its General-Secretary from 1935 to 1977. Goonewardene was a key figure in both the Indian independence movement and the Sri Lankan independence movement. He was designated as a National Hero of Sri Lanka for his leadership in the independence movement, and his efforts are celebrated each year on the Sri Lankan Independence Day.