Suthep Thaugsuban, former secretary-general of the People's Democratic Reform Committee (PDRC), left the monkhood in a ceremony at Wat Traidhammaram in Muang district of Surat Thani province on Tuesday morning.
Other former Democrat MPs who left monkhood at the same time included Mr Suthep's younger brother Chen Thaugsuban and Chinaworn Boonyakiat.
The ceremony began at 6am and was presided over by Phra Dhamvimolmolee, chief of the 16th Monastic Region.
After the ceremony, Mr Suthep went with some former Democrat Party MPs to Suan Mokkhaphalaram in Chaiya district of Surat Thani to give alms to monks who yesterday entered monkhood in a mass ordination in honour of His Majesty the King.
Mr Suthep said he would now concentrate on the activities of the People's Democratic Reform Foundation.
The foundation would pursue the PDRC's six-point proposal for reforms, he said.
Mr Suthep led PDRC street protests against the Yingluck Shinawatra government until the military coup on May 22 last year. He entered monkhood shortly after the coup.