Douglas Carswell said Britain's vote to leave the EU made it the right time to leave the party, adding that he did so "amicably".
Writing on his website, Mr Carswell said it was a "job done" on Brexit, and he would now focus on constituency issues.
He said: "Like many of you, I switched to UKIP because I desperately wanted us to leave the EU.
"Now we can be certain that that is going to happen, I have decided that I will be leaving UKIP.
I will not be switching parties, nor crossing the floor to the Conservatives, so do not need to call a by election, as I did when switching from the Conservatives to UKIP.
"I will simply be the Member of Parliament for Clacton, sitting as an independent.
"I will leave UKIP amicably, cheerfully and in the knowledge that we won."
Although the loss of its only representative in Parliament will be seen as a blow by some, there are many within UKIP who will be glad to see the back of him.
Moments ago, the Leave.EU campaign tweeted a poll asking if Mr Carswell was a "Tory plant" all along.
The tweet read: "Duplicitous Douglas Carswell, who supports mass immigration, is finally out of UKIP. Was he a Tory plant all along?"
A UKIP source told the Express: "Champagne corks are popping."
Mr Carswell has also been involved in a bitter war of words with former UKIP leader Nigel Farage for several years.
The pair's latest row came only last month, when Mr Farage publically called for Mr Carswell to go.
He wrote in the Telegraph: “I think there is little future for Ukip with him staying inside this party. The time for him to go is now.”