The Social Democratic Party of Lithuania (Lithuanian: Lietuvos socialdemokratų partija, LSDP) is a social-democratic political party in Lithuania. It is the longest-existing party in Lithuania, having been founded as an underground Marxist organization in 1896. During the period of Soviet occupation the party was forced into exile, emerging once again in Lithuania in 1989.
The party's chairman since 2009 is Algirdas Butkevičius. The party led a minority government in the unicameral Seimas, Lithuania's Parliament from 2004 to 2008. The party is a member of both the Party of European Socialists and the Socialist International.