Red-shirt leader Nattawut Saikuar has returned to Pheu Thai as the party looks to increase its following, aiming for a big win in the coming general election.
Pheu Thai reintroduced Mr Nattawut to the party on Wednesday and senior adviser Paetongtarn Shinawatra commended him for his decision, saying it would benefit the party.
"His return will help strengthen the party," the chief of the party's inclusion and innovation advisory committee said.
The daughter of self-exiled former prime minister Thaksin is widely expected to be the party's candidate for prime minister at the general election.
Mr Nattawut, who is secretary-general of the United Front for Democracy against Dictatorship, said he would consult party leaders on what role he would play.
His first activity with Pheu Thai will be on Saturday, when he will join a road trip to Si Sa Ket, home province of two of the seven Pheu Thai MPs who rebelled and did not vote the party line during the recent budget bill debate.
They are Jaturong Pengnoraphat and Theera Traisaranakul, who are both expected to join the Bhumjaithai Party for the election.
"We will explain to the people the importance of choosing their representatives wisely when voting," Mr Nattawut said.
Mr Nattawut left Pheu Thai to join the now-disbanded Thak Raksa Chart Party for the 2019 general election.