Party members voted by 230 to 183 in support of nuclear power in “limited” circumstances after Energy and Climate Change Secretary Ed Davey told their conference in Glasgow it would be “reckless” to rely on coal and gas, along with renewable sources of energy.
The party has always been opposed to nuclear power but rejected an amendment to a policy motion, which will dictate the Lib Dem manifesto at the 2015 general election.
There were also concerns about the cost of new nuclear power stations but Mr Davey told the conference he would not allow the price of projects to get out of control.
Earlier, environmental campaigners urged the party to reject nuclear power in an effort to restore its green credibility.
Mr Davey said: “When I have listened to pro-nuclear Liberal Democrats over the years, there is one argument I have found increasingly difficult to answer and that is the climate change argument.
“Climate change poses a real and massive danger to our planet. Not keeping a genuinely low source of electricity as an option is reckless when we don’t know the future.
“We are going to need vast amounts of low carbon electricity to tackle climate change. Why? Because if our carbon capture and storage plans don’t work, we may have to replace all fossil fuels for electricity generation, that is about 60% of all generation.
“If we do that without nuclear, you will need to replace about 85% of electricity generation. That’s huge.”
Mr Davey said he was absolutely determined not to sign any contract for new nuclear power stations which relied on public subsidy, adding he had changed his mind on nuclear power because of climate change.
He added: “New nuclear must be cost-competitive. We will not repeat the history of mistakes on nuclear.”