Jurin Laksanawisit announced his resignation as leader of the Democrat Party late on Sunday night, accepting responsibility for the party's rout in Sunday's general election.
About 5.30pm on Sunday, Mr Jurin thanked voters who supported his party in the election after the voting was closed.
At 11.30pm, he posted on the Democrat Party's LINE group, saying:
"I would like to take this opportunity to congratulate those who have been elected and extend my moral support to all candidates.
"Also, I would like to profoundly thank the party secretary-general (Chalermchai Sri-on), Mr Banyat (Bantadtan), Mr Abhisit (Vejjajiva), deputy party leaders, members of all working groups, executive committee members and party members for having worked for the party to the best of their ability.
"To show responsibility for the party's election results, I hereby announce my resignation as the party leader. I would like to ask you all to continue working for the party. I myself will stay on with the party, regardless of in what position."
The latest update of the unofficial results of the May 14 election shows the Democrat Party had won 25 House seats - 22 from constituencies and three from the party list.
After the 2019 general election, the party had 52 MPs - 33 from constituencies and 19 from the party list. It then joined the coalition government led by Palang Pracharath.
Counting of constituency seat votes was 99% completed, according to the Election Commission website on Monday morning.