The Harju County Social Democratic Party (SDE) branch has elected MP Jaak Aab as its chair.
Aab, a former minister, recently quit the Center Party to join SDE, part of a wave of defections from that opposition party to several other Riigikogu parties.
Harju County is the most populous of Estonia's counties.
Jaak Aab was unanimously elected, while current incumbent, Jüri Ross, will continue on the regional SDE board in Harju County.
Aab, 63, was a member of the Center Party for nearly 30 years, having joined in September 1994.
He formally left Center on January 5, over differences with party leader Mihhail Kõlvart, joining SDE five days later.
SDE is to hold a congress in Paide, often a venue for party meetings given its central location, on March 16.
Harju County's SDE chapter says it will back Lauri Läänemets staying on as party leader, and will back another new arrival from Center, Tanel Kiik, as a party vice-chair.
The Harju County SDE branch will also support Eve Kislov, who heads up SDE's women's association, as party vice-chair.
Aab announced in a press release that he is: "Grateful from the bottom of my heart for this expression of trust, since I have only been in the party for a month."
"I will attempt to justify it in every way, as the position of chair is also a major responsibility. We have some great and enterprising people in the region, with whom you can 'go prospecting' with at any time," he went on.
The major challenges currently facing SDE are June's European Parliament election, and next year's local government election, Aab added.
Editor: Urmet Kook, Andrew Whyte