Mr Reckless, a member of the Welsh Assembly, sensationally left the Conservatives in 2014 for Nigel Farage’s party causing David Cameron to apparently brand him a “fat ****”.
His defection was a coup for Mr Farage, who also poached Clacton MP Douglas Carswell at the same time, in a significant blow to the Conservatives.
But now Mr Reckless has parted ways with UKIP to join the Conservative Group in the Welsh Assembly, saying leaving the EU was “job done”.
He said: “I campaigned all my adult life for on to leave the EU. Now we are, under a Conservative Government. I support Theresa May, and her team of Brexit ministers to deliver.
“As a country, it is right that we seek a sensible settlement with the EU, rather than no deal. The UK Conservative government is getting us out of the EU and I want to back it.
“In Wales, the Conservatives are the main opposition to Labour. It is time that was properly reflected in the Assembly.”
UKIP has said Mr Reckless must now surrender his seat on the Welsh Assembly to another UKIP candidate from the regional list.
Former UKIP leader Nigel Farage said: “To be elected on a list and then to resign from the party is a dishonourable thing to do, and I thought better of him.”
It comes after Mr Carswell quit the party to become an independent MP following a bitter and public two-year battle with Mr Farage.
The Conservative Party stressed that Mr Reckless was not “rejoining” the party but would simply be treated as a member of the group in the Welsh Assembly.
When Mr Reckless left the Tories he triggered a by-election in Rochester and Strood, which he won putting two UKIP MPs in the House of Commons – him and Mr Carswell.
He lost his seat in the 2015 election but won his seat on the Welsh Assembly in 2016.
UKIP Chairman Paul Oakden said that Mr Reckless should not be allowed to retain his assembly seat because he won it under the UKIP mantle and it should go to another party candidate.
He said: “We are disappointed to hear that Mark Reckless has decided to leave UKIP in order to join the Conservatives in the Welsh Assembly.”
Former UKIP donor Arron Banks said Mr Reckless was a “weak man that fell into bad company”.
He claimed the defections of both Mr Reckless and Mr Carswell had been coordinated by the Conservative MEP and chief Brexiteer Daniel Hannan.