Pirapan Salirathavibhaga resigned from the Democrat Party on Monday - a decision that also terminated his status as a party-list MP.
He submitted a letter, dated Dec 9, to the Election Commission's political party registrar, formally informing the poll organising agency of his resignation from the party, effective immediately.
Mr Pirapan said in the letter that he had told Democrat leader Jurin Laksanawisit and the party's registrar of his decision.
He did not disclose the reason he decided to suddenly part ways with the party, a decision that automatically terminated his position as a party-list MP.
Mr Pirapan lost to Mr Jurin in the contest for the party leadership in May, after Abhsit Vejjajiva resigned following the party's poor showing in the general election in March.
Mr Pirapan is the second prominent Democrat member to quit the country's oldest party recently. Warong Dechgitvigrom resigned last month to join the Action Coalition for Thailand under former Democrat secretary-general Suthep Thaugsuban.
Pimrapee Phanwichatikul is in line to replace Mr Pirapan as the new Democrat MP under the party-list formula.