Lithuanian Popular Socialist Democratic Party (Lithuanian: Lietuvos socialistų liaudininkų demokratų partija) or LSLDP was a political party in interwar Lithuania. It was established in 1917 and in 1922 it was merged with another party. The party was established in Voronezh by Mykolas Sleževičius, Felicija Bortkevičienė, and others, mostly from the left wing of Lithuanian Democratic Party.
In 1917 Jonas Vileišis, member of the party, was elected into the Council of Lithuania. The party adopted its statute in 1919; it proclaimed that the main goal of the party was to make land a common property of the people. It won nine seats of out of 112 in the Constituent Assembly of Lithuania and six seats out of 78 in Seimas elections of 1922. The party published several political newspapers as well. On December 3, 1922 Lithuanian Popular Socialist Democratic Party merged with Lithuanian Peasants Union and formed a new political party known as Lithuanian Peasant Popular Union.