Former Democrat Party member Kanok Wongtrangan joined Chartthaipattana on Monday to be another "thinker" for his new party.
Mr Kanok, a former party-list MP, applied for membership and was welcomed by Chartthaipattana leader Varawut Silpa-archa and other executives.
The former long-time Democrat said he was impresed by Mr Varawut's environmental policies and expected to join him on his campaign drive.
Mr Varawut is the natural resources and environment minister in the coalition government.
The Chatthaipattana leader welcomed his new member and said Mr Kanok would be another think-tanker, helping map out strategies for the party, and promised to include him on the party list at the coming general election.
Mr Kanok was a Democrat member for two decades before deciding to jump ship.