The Harmony has been an associate member of the PES since June 2015 but now has become the 34th full member of the PES.
"I am delighted to welcome the Harmony as the thirty-fourth full member of the PES family. With their progressive political program at both national and European level, their membership will strengthen our progressive family still further. I look forward to cooperating closely with the Harmony in the future," said PES President Sergei Stanishev.
The PES comprises national-level political parties from all EU member states plus Norway. Its political group in the European Parliament is the Progressive Alliance of Socialists and Democrats (S&D).
The pact with the party alliance coincides with a major policy shift for the Saskaņa party which was long criticized for its long-time cooperation agreement with Vladimir Putin's United Russia party.
In early October, the party head and Riga mayor Nils Ušakovs revealed to LTV that his party had ditched the agreement with United Russia and attempted to downplay the importance of the move.
The agreement had lapsed when Saskaņa joined the EU-level alliance of parties known as the Party of European Socialists as "traditionally, parties that are in this group base their cooperation with parties in third countries in a centralized manner... so our agreement with United Russia which lapsed in 2016 has lost its applicability," Ušakovs said.