Democrat leader Abhisit Vejjajiva conceded the election on Sunday night after unofficial results made it clear the Pheu Thai Party had won an overwhelming victory.
"I would like to thank all Thais who voted for the Democrat Party," the outgoing prime minister said.
He said Pheu Thai's victory was clear from the unofficial election result.
Mr Abhisit said he would like to congratulate Pheu Thai's top party list candidate Yingluck Shinawatra as the next and first female prime minister.
"In the past two years, I have committed myself to overcome the crisis and I would like to thank Thai people for moving the country to the current position.
"I wish Pheu Thai will solve the people's well-being," Mr Abhisit said.
He said the Democrats are ready to be opposition.
On the question of his party leadership, Mr Abhisit said he already had an answer in his mind but would rather wait to see the final figures of election results first.
Later at about 7.50pm, Ms Yingluck gave her address to thank Pheu Thai's supporters.
