Justice Secretary Michael Gove has said Britain would be "freer, fairer and better off outside the EU".
Michael Gove said his decision to defy his friend and ally the prime minister and back the leave campaign was the most difficult of his career.
But he said the EU had "proved a failure on so many fronts" and he felt compelled to put his political convictions before loyalty to the PM.
He is the most high-profile minister to declare for out so far.
Four other cabinet ministers - Iain Duncan Smith, Chris Grayling, John Whittingdale and Theresa Villiers - have signed up to the Leave campaign.
Home Secretary Theresa May - who out campaigners had hoped would join their cause - has backed the campaign to remain in the EU.
In a lengthy statement on his reasons for joining the leave campaign, Mr Gove said: "I believe our country would be freer, fairer and better off outside the EU. And if, at this moment of decision, I didn't say what I believe, I would not be true to my convictions or my country.
"By leaving the EU we can take control. Indeed, we can show the rest of Europe the way to flourish.
"Instead of grumbling and complaining about the things we can't change and growing resentful and bitter, we can shape an optimistic, forward-looking and genuinely internationalist alternative to the path the EU is going down."
He said he had wrestled for weeks "with the most difficult decision of my political life".
"I was encouraged to stand for Parliament by David Cameron and he has given me the opportunity to serve in what I believe is a great, reforming government.
"I think he is an outstanding prime minister. There is, as far as I can see, only one significant issue on which we have differed. And that is the future of the UK in the European Union.
"It pains me to have to disagree with the prime minister on any issue. My instinct is to support him through good times and bad.
"But I cannot duck the choice which the prime minister has given every one of us."
'Courageous and principled'
Mr Gove said it was "hard to overstate the degree to which the EU is a constraint on ministers' ability to do the things they were elected to do, or to use their judgment about the right course of action for the people of this country".
Addressing claims by Mrs May and the prime minister that remaining in the EU was the best option for Britain's future security, he said: "Far from providing security in an uncertain world, the EU's policies have become a source of instability and insecurity."
Mr Cameron has warned that leaving the European Union would be a "leap in the dark", as he urged voters to back his reform deal.
The PM said: "Those who want to leave Europe cannot tell you if British businesses would be able to access Europe's free trade single market, or if working people's jobs are safe, or how much prices would rise. All they're offering is a risk at a time of uncertainty - a leap in the dark."
Mr Gove's decision was welcomed by out campaigners, with UKIP MEP Roger Helmer tweeting: "Seems that Michael Gove will campaign for #Brexit. Well done that man. Courageous and principled."
There has been increasing speculation in recent days that Mr Gove would argue for an EU exit, with some commentators suggesting he would not lead the campaign.
Leave campaigners have been pinning their hopes on landing a big name to go up against Mr Cameron - with London Mayor Boris Johnson, who is yet to reveal which side he will back, still their top target.