The Remain backing Streatham MP left Labour in February in protest to Jeremy Corbyn’s leadership and the party’s Brexit position.
He was one of the founding MPs of Change UK but resigned with five others last month.
Like Change UK, the Liberal Democrats oppose Brexit and are campaigning for a second referendum.
However, Mr Umunna’s previous tweets on the party have reemerged, in 2013 he accused them of “betraying students” over tuition fees before dismissing Vince Cable’s proposals to increase the minimum wage: “You can’t trust a word the Lib Dems say.”
In 2012 he also accused of minister Danny Alexander of being “the latest LibDem who thinks he can hoodwink the public-pose” by portraying themselves as anti-Tories despite, as he put it, “they signed up to all his failed policies”.
Admittedly, these tweets came several years ago, when Mr Umunna was a Shadow Cabinet minister and the Liberal Democrats formed a coalition government.
However, posts since the EU referendum have continued to tear into the Liberal Democrats.
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After Tony Blair suggested an electoral alliance in the snap 2017 election encompassing Remain candidates, Mr Umunna dismissed the idea, writing on Facebook: “I could never ever countenance suggesting voters support Liberal Democrat candidates.”
He then described their record in power as “utterly unforgivable”.
In the run-up to the vote, he used to Twitter to accuse both the Conservatives and Liberal Democrats of pushing “our schools, NHS and local government” to a point where they were “starved of funding”.
In a 2015 Labour Party press release he was quoted as saying: “Vince Cable can’t run away from his record”.
He has now joined the Mr Cable led party and will stand as their candidate in the next general election despite insisting in 2017: “I can’t forgive what they’ve done to my area.”
Mr Umunna has not yet called a by election and will be the 12th Liberal Democrat MP.
He is also the fourth of their MPs to represent a London constituency after Tom Brake (Carshalton and Wellington), Mr Cable (Twickenham) and leadership candidate Sir Ed Davey (Kingston and Surbiton).
According to the party website: “Every vote for the Liberal Democrats is a vote to stop Brexit and stay in the European Union.”