Stewart Jackson, a prominent Brexit supporter and ex-Parliamentary Private Secretary to Mr Davis, said he was "hugely honoured" to take up the post at a "historic juncture".
He lost the Peterborough seat he had held for 12 years in the June general election and pinned the blame on Theresa May's manifesto - and also criticised her for not getting in touch with him after his defeat.
In the days following he described the manifesto as "shockingly bad ... lacking passion, it was bland, it lacked vision ... above everything else a real lost opportunity".
Mr Jackson confirmed his new job as special adviser (SpAd) and chief of staff at the Department for Exiting the European Union in a message on Twitter.
He said: "Hugely honoured to be appointed as SpAd & Chief of Staff to Sec State DExEUgov.
“Fantastic job at historic juncture. Now signing off Twitter."
Gavin Barwell, another Tory who lost his seat at the election, was appointed as Ms May's chief of staff in June after her two most senior aides quit in the wake of the election which saw the Prime Minister lose her Commons majority.
Mr Jackson courted controversy in June when he reportedly contacted a former constituent who had been critical of him on Facebook to call him a "thick chav", days after losing his Peterborough seat.
In 2011, he criticised a pregnancy provider for being "morally squalid" after it launched a service to text women reminders about upcoming abortion appointments.
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