It was earlier speculated that Yinglak Shinawatra, a sister of former prime minister Thaksin Shinawatra, would be elected as the new Pheu Thai leader.
The party election was held at the party head office at 10 am.
Former Prime Minister Somchai Wongsawat and his wife, Yaowapha Wongsawat, left their house at 9:30 am to observe the election of the new Pheu Thai board.
The party assembly voted unanimously to retain the same number of 13 party executives before electing the new board.
Caretaker party registrar Kamol Bandaiphet nominated Yongyuth to the post of party leader without anyone nominating another candidate to contest against him.
Yinglak, who was sitting in the front row, was apparently surprised that her name was not nominated.
She rose from her seat and walked to the back of the room to talk to a party member after her name was not nominated.
When realising that she was being watched by reporters, she and the party member went inside a room behind the meeting room immediately.