Move Forward MPs resolved on Monday the party would not join a coalition government with the Palang Pracharath (PPRP) or United Thai Nation (UTN) parties.
The party announcement after the meeting said that PPRP was led by Gen Prawit Wongsuwon and UTN had Gen Prayut Chan-o-cha as its prime ministerial candidate.
Both were key figures in the coup d'etat on May 22, 2014, and there was a push to prolong their hold on power.
Voters showed through the May 14 general election that they wanted different people in government and MFP, as the election winner, was determined to form a new government and ensure they did remain in power.
Any attempt to form a government that went against the people's decision would erode their faith in democracy, the announcement said.