Somchai Srisutthiyakorn, a former election commissioner, says he will join the Democrat Party.
He did not say whether he would run as an MP candidate for the country's oldest political party, only that he would apply as a member.
Mr Somchai, who was sacked by Prime Minister Prayut Chan-o-cha early this year, made the announcement on Saturday.
In a Facebook post, he thanked friends from "seven or eight parties" who had asked him to join them.
Since the law allows a person to be member of only one party, Mr Somchai said he had to choose.
"All of these parties are pro-democracy. If they don't support democracy, they won't run in the poll," he wrote.
"I can't pick the best party in the eyes of everyone. That's impossible, given each has different perceptions and values. But how do I choose so people will benefit the most in my view? The answer to this led to the final decision.
"On Sunday, I'll apply to become a member of the Democrat Party."
He also wrote that the party he joins must not support dictatorship.
Mr Somchai is an acting dean of the School of Political Science and Laws at Walailak University in Nakhon Si Thammarat province after leaving the Election Commission in March.
Earlier in his career, he was a lecturer and later deputy dean at Thammasat University's Faculty of Political Science.
He was later involved in politics as a leading member of the Open Forum for Democracy, monitoring elections.
He was chosen as an EC commissioner in 2014 but sacked in March this year by Gen Prayut, in his capacity as head of the National Council for Peace and Order (NCPO).
