MP Heidi Allen, who quit the Conservative Party earlier this year, has joined the Liberal Democrats.
The MP for South Cambridgeshire left the Conservatives in February over its Brexit policy and other issues.
She subsequently became the leader of the fledgling Change UK but left after the party's failure to win any seats in the European elections.
She is the fourth ex-Tory to join the Lib Dems in recent months, after Sarah Wollaston, Philip Lee and Sam Gyimah.
Her move means the Liberal Democrats now have 19 MPs, eight more than at the start of the year.
In a statement, the 44-year old said she would fight the South Cambridgeshire seat for her new party at the next election and had been "bowled over" by the support she had received.
Ms Allen said the Conservatives and Labour had both "moved to the extremes" and it was only the Liberal Democrats which now occupied the "liberal centre ground" of British politics.
'Part of a team'
The MP, who has been sitting as an independent in Parliament for several months, said she could be "stronger and more effective" in her opposition to Brexit as "part of a team".
"Now is the time to stand shoulder to shoulder with, not just alongside, those I have collaborated and found shared values with," she said.
"As we face the monumental task ahead of stopping a damaging Brexit, healing the rifts in the UK and rebuilding the UK, there is only one party with the honesty, energy and vision to do that."
Welcoming the party's latest new recruit, the Lib Dem leader Jo Swinson said Ms Allen had "long been an ally" in the fight to stop Brexit.
She said her arrival showed that the Lib Dems were the "strongest party of Remain".
Ms Allen was first elected to Parliament in 2015, having previously worked for her family business as well as ExxonMobil and Royal Mail.
She caused ripples in her maiden speech in the Commons, decrying tribalism in Parliament and attacking elements of the government's welfare policy.
After walking out of the Tories earlier this year, she caused controversy by suggesting that if she and other defectors did their job, the Conservatives would no longer "need to exist".
But Change UK only managed to win 3.4% of the vote in May's European elections after reportedly refusing to co-operate with the Lib Dems and other anti-Brexit parties.
She is the fifth of the 11 founding members of Change UK to join the Lib Dems - following Chuka Umunna, Luciana Berger, Angela Smith and Sarah Wollaston.
She faces a tough task in retaining her seat at the next election, which has been a safe Conservative seat since its creation in 1997.
In 2017, the Lib Dems came in third place in the constituency - more than 20,000 votes behind the Tories and more than 5,000 votes behind Labour.
However, the Lib Dems took control of South Cambridgeshire council in last year's local authority elections. The area voted strongly to remain in the 2016 Brexit referendum.