In a secret ballot election, Palionis, 41, secured 11 votes against six votes for Juozas Olekas, his only competitor.
Palionis, who came in third in the LSDP chairman election last April, will replace Irėna Šiaulienė, deputy speaker of the Seimas, as the head of the Social Democrats’ political group.
Algirdas Butkevičius, a former LSDP chairman and a former prime minister, announced on the eve of the election that he would not run for the post.
Palionis, who has a BA in accounting and audit, last autumn was elected to the Seimas for a second term. He is a member of the parliament’s Anti-corruption Commission and the Commission for Ethics and Procedures.