The UK Green Investment Bank, one of the major policies of the coalition government, will begin operating in April 2012, according to Nick Clegg.
He said that some of the bank's early targets would be "offshore wind, waste and non-domestic energy efficiency".
The deputy prime minister added that legislation would ensure the independence of the institution.
In March, MPs warned that the bank would suffer if ministers compromised on their promises.
"Investing in a greener future is an ambition that combines an ethical duty and an economic duty," Mr Clegg said.
He explained that the first investments would be able to be made from April 2012, which was likely to be before the necessary legislation had made it to the statute books.
The Liberal Democrat leader said that more details about how the bank would operate before it was enshrined in law would be provided by Business Secretary Vince Cable on Tuesday.
He added that the establishment of the bank would be a "world first".
"Most countries have a development bank, but the UK will be the first country to have a national bank dedicated to the green economy," he said.
In the Budget, the government guaranteed that the bank would receive an initial capital of £3bn.
Ministers hoped to raise the funds by selling off assets, but Mr Clegg added that the Treasury had underwritten the scheme to meet any shortfalls.
"The initial £3bn capital by 2014/15 should enable the bank to catalyse an additional £15bn of investment in the green infrastructure," he said.
Back in March, a report by MPs said that the UK could lose out on hundreds of billions of pounds in green investment and fail to meet climate change targets if the government compromised on its plans for the green bank.
The Environmental Audit Committee said the institution had to be able to raise additional capital from investors. The findings were published amid concerns that the coalition was planning to water down its plans for the bank.
The latest announcement, made by Mr Clegg during a speech in central London, comes shortly after the coalition was criticised by green groups for failing to live up to the pledge of being the "greenest government ever".
On the first anniversary of the Conservatives and Lib Dems joining forces, Friends of the Earth published a review that said there had been little or no progress on 55 out of 75 policies.
The study, carried out by Jonathon Porritt, former head of the Sustainable Development Commission, said the government had scrapped a planned rise in aviation tax, failed to create a green investment bank with immediate borrowing powers and had watered down schemes promoting small-scale renewable electricity.
Responding to Mr Clegg's speech, Friends of the Earth's senior economy campaigner Simon Bullock welcomed the news that ministers planned to introduce legislation to protect the bank.
"This will give businesses the reassurance they are looking for to invest in British renewable energy jobs and industries," he told BBC News.
"Using the bank to cut energy waste from homes will also help to tackle fuel poverty and climate-changing emissions.
"But there is still room for dinosaur forces in the Treasury to hamper the bank's effectiveness - the government needs to say legislation will happen next year at the latest, and make clear the bank can borrow from the capital markets from the outset."
Matthew Spencer, director of the Green Alliance, also voiced concern about how the reported division within government was affecting green policies.
"Its excruciating watching the birth of the green bank - every step forwards generates a new paternity dispute," he told BBC News.
"We now know it will be set up in legislation but not when, that it will have an interim governance structure, but not what that is.
"It's a strange way to run a green growth strategy, because Whitehall tussles for control undercut what could otherwise be a strong signal to investors."
But Paul King, chief executive of the UK Green Building Council, supported the latest details outlined by Mr Clegg, saying that the Green Investment Bank would play a central role in the green economy.
"Today's announcement is tremendous news for anyone with an interest in creating a strong, independent Green Investment Bank, which has the capacity to kick-start the step change needed to create a low carbon economy," he said.