Douglas Carswell said he made the decision to "shake-up" the Westminster "clique".
He's resigned from parliament but will stand for election as a UKIP candidate in a by-election.
If he wins he'll be the first UKIP MP.
"Douglas Carswell won the Harwich seat from Labour in 2005 and held it, renamed Clacton, in 2010 with a majority of 12,068. He is known in the Commons as right winger strongly opposed to British membership of the EU.
"Clacton is a relatively deprived constituency. One third of its residents are pensioners. This and the size of his majority suggest that he could win the by- election and give UKIP their first MP.
"Carswell's defection could encourage other Conservative Europhobes to divide the party damaging its election prospects next year, making the promised In/Out referendum in 2017 less likely."
He said Prime Minister David Cameron was "serious about the change we need".
He added he wanted "fundamental change in British politics" and claimed only his new party, UKIP, could deliver that.
"This hasn't been an easy decision. I have been a member of the Conservative Party for all my adult life. It's full of wonderful people who want the best for Britain
"My local Conservative Association in Clacton is thriving. It brims with those I'm honoured to call my friends.
"The problem is that many of those at the top of the Conservative Party are simply not on our side. They aren't serious about the change that Britain so desperately needs.
"Of course they talk the talk before elections. They say what they feel they must say to get our support when they want our support, but on so many issues - on modernising our politics, on the recall of MPs, on controlling our borders on less government, on bank reform, on cutting public debt, on an EU referendum - they never actually make it happen."
A Conservative party spokesman said: "This is a regrettable and frankly counterproductive decision.
"As he's said, the only way to get an referendum on the EU is to return a majority conservative government.
"The Conservative party will contest the forthcoming by-election vigorously to make the people in Clacton have a strong Conservative voice in this parliament and the next"