In a statement released on his official Facebook page, the former London Mayor, who announced his intention not to run for the leadership last week, said the energy minister had "the zap, the drive and the determination essential for the next leader of this country".
He said: "She has long championed the needs of the most vulnerable in our society.
"She has a better understanding of finance than almost anyone else in Parliament. She has considerable experience of government.
"She is level headed, kind, trustworthy approachable and the possessor of a good sense of humour".
He said she was the best placed candidate to lead the UK through the EU negotiations and "help forge a great post Brexit future for Britain and Europe".
The endorsement has been seen as retaliation for Justice Secretary and fellow Vote Leave campaigner Michael Gove, whose surprise announcement that he would run for the leadership effectively ended Mr Johnson's own ambitions.
In an stinging attack on the man who had campaigned by his side during the referendum Mr Gove said he did not believe Mr Johnson could "provide the leadership or build the team for the task ahead".
The debate between the various candidates has been more starkly divided over the issue of EU citizens currently living in the UK, with Ms May coming under attack from fellow MPs for her failure to guarantee the status of EU citizens living in the country.
Tomorrow the Conservative Parliamentary Party will cast their first ballot to decide their next leader after David Cameron announced his resignation following the EU referendum.
Michael Gove and Work and Pensions Secretary Stephen Crabb are trailing on 25 and 22 backers respectively.
Unlike the Labour leadership elections, Conservative MPs vote in secret ballots until there are two candidates left to be put to the wider party membership.
A leader is expected to be in place by September.
It comes as Ms Leadsom was criticised by senior Conservatives for a "car crash" performance in the first hustings in front of MPs in Parliament.
The MP for South Northamptonshire came unstuck after she failed to distance herself from Arron Banks – the billionaire who bankrolled both Ukip's general election campaign and the unofficial Leave.EU campaign.
The Leave.EU said Ms Leadsom was the "clear frontrunner" in the race and the "breakout star of the Leave campaign".
One unnamed cabinet member told the Huffington Post: “When you’re having to say that you’re not Ukip at a hustings to be leader of the Conservative Party, then you are in trouble - it was a car crash.”
A backbencher said she “spent too long on Europe and started going on about attachment theory of babies, it was all over the place.”
Another Tory said when Ms Leadsom started talking about babies "she lost the room".
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