The Social Democratic Party of Lithuania (Lithuanian: Lietuvos socialdemokratų partija, LSDP) is a centre-left and social democratic political party in Lithuania. Founded as an underground Marxist organisation in 1896, it is the oldest extant party in Lithuania. During the Soviet occupation, the party ceased to exist. The party reemerged after the fall of the Soviet Union and the end of communist rule in 1989.
The party led a government in the unicameral Seimas, Lithuania's parliament from 2001 to 2008 and from 2012 to 2016. It has been the ruling party of Lithuania since 2024. The party is a member of the Party of European Socialists (PES), Progressive Alliance, and Socialist International.
