The Rochester and Strood MP accused the Tory leadership of failing to keep its promises on Europe, the economy and immigration, and said the anti-Brussels party offers a "fresh start".
Appearing on stage to a rapturous reception at the eurosceptic party's conference in Doncaster, South Yorkshire, Mr Reckless told activists voters felt "ripped off and lied to".
"Today I am leaving the Conservative Party and joining Ukip," he said.
Mr Reckless told the conference he answers to his constituents and wants to seek their permission to change allegiance.
"They are the boss and if I am to seek to represent them under different colours, I hope in a party that is closer to their values, then I shall ask their permission," he said.
"So I will resign my seat in Parliament, trigger a by-election and, your national executive allowing, stand for Ukip."
The Conservatives described the defection as "completely illogical" but Mr Reckless claimed the party's leadership was "part of the problem that is holding our country back".
The defection, which comes on the eve of the Conservative Party conference in Birmingham, is a fresh blow to David Cameron and will fuel speculation over the possibility of other Tories jumping ship.
As he was introduced on stage by a clearly delighted Nigel Farage, the crowd broke out into whoops and cheers with some activists taking to the aisles to dance in celebration.
Mr Farage told BBC News that he would do "whatever it takes" to get Mr Reckless elected.
"This man has shown huge courage. He has thrown his lot in with us and we will do everything we can to get him elected."
Asked about other possible defections, he replied: "We have these conversations. Of course there are Conservatives I am talking to but there are Labour people too. There are Labour people who are deeply frustrated with Ed Miliband's leadership."
Mr Reckless said he had endured a number of sleepless nights over the decision, telling the conference he had been a Conservative "as long as I can remember".
But he told Ukip activists that too many election pledges had been broken during the course of the parliament.
"People feel ignored, taken for granted, over taxed, over regulated, ripped off and lied to," he said.
He dismissed the Prime Minister's promise of an in/out referendum on membership of the European Union.
"I'm afraid I have reluctantly reached the view that he is doing so purely as a device," he said.
"He has already pre-ordained his intended outcome, namely continued membership of the EU on something very close to the present terms.
"Everything else is for show.
"What the Prime Minister has in mind, and it's not even a secret at Westminster, is modelled on what Harold Wilson did in 1975 - a bogus renegotiation followed by a loaded referendum."
He said the Government had added "more to our national debt than even Labour managed in 13 years" and criticised the decision to promise to retain a funding formula that favours Scots over the English.
Mr Reckless said constituents needed to believe that Britain had control over who comes into the country and in what numbers.
"The insanity of our immigration rules means that a second generation Briton wanting to bring granny over for a wedding, still less if they want to get married to someone from abroad themselves, will face huge difficulties, yet they will see an open door to immigration to anyone from the European Union," he added.
Mr Reckless faces a tough battle to return to Westminster under the Ukip banner. At the 2010 general election he held the Rochester and Strood seat for the Conservatives with half of the vote and Ukip did not even field a candidate.
He said: "If we can win in Rochester and Strood as well as in Clacton and perhaps here in South Yorkshire, then we can show that Ukip can break through across the country and we will show once and for all that a vote for Ukip is a vote for Ukip," he added.
Ukip's prospective parliamentary candidate in the constituency, Mark Hanson, had been unaware of the plans to install Mr Reckless in the seat but told conference that he was happy to quit.
Before hitting the campaign trail for the Kent seat Ukip is fighting to secure election for Mr Carswell in Clacton and John Bickley in Heywood and Middleton, with voters going to the polls in both by-elections on October 9.
Mr Farage told the conference that research shows Mr Carswell is on course to win with 56% of the vote, pushing the Conservatives into second on 24%.
The poll by Conservative peer Lord Ashcroft suggests that Ukip will also take seats in South Thanet - where the Ukip leader is standing - Boston and Skegness, Eastleigh and Thurrock.
Andrew Mackness, chairman of Rochester and Strood Conservatives, said: "I am astonished and disgusted by Mark Reckless' decision - only 48 hours ago he proclaimed his support for the Conservatives and their plans for a referendum on Europe and he gave me assurances he wouldn't defect.
"He has misled the hardworking people of Rochester and Strood who voted for him. It is only the Conservatives who can deliver a referendum on Europe with a vote for Ukip being a vote for Ed Miliband."
A Conservative spokesman said: "Mark Reckless' decision to join Ukip is completely illogical. He says he wants action on a European referendum, tax and immigration. The only party capable of delivering on these issues is the Conservative Party - and a vote for Ukip is a vote for Ed Miliband."