He starts on 1 February, providing "perspectives on European politics and policy, Chinese economic reform, and trends..." BlackRock said.
Mr Osborne, whose salary was not disclosed, will remain an MP, but will not be allowed to lobby the government.
He said it was a chance to work with one of the world's most respected firms and a major employer in Britain."
Mr Osborne added: "BlackRock wants better outcomes for pensioners and savers - and I want to help them deliver that."
He joins his former chief of staff, Rupert Harrison, who is a senior strategist for the US investment firm.
"George has a unique and invaluable perspective on the issues that are shaping our world today," said Laurence D. Fink, chairman and chief executive of BlackRock.
Mr Osborne, a prominent campaigner to keep the UK in the European Union, was sacked from the government after Theresa May became prime minister following the Brexit vote.
He became the UK's youngest finance minister in more than 100 years when he was appointed in 2010 to spearhead an austerity drive by the Conservative-led government following the global financial crisis.
In November, he defended earning Â£320,000 in speaking fees in a single month after leaving government.
His appointment follows other political heavyweights joining financial firms. Just this week, former foreign secretary William Hague joined Citigroup as an adviser.
Former prime minister Gordon Brown sits on the global advisory board at investment manager Pimco, and his predecessor Tony Blair joined JP Morgan in 2008 shortly after leaving office.
BlackRock manages more than Â£4 trillion ($5trillion) worth of investments and pension funds worldwide and is often consulted by governments.
Mr Osborne will advise senior BlackRock executives on macroeconomic policy, providing - what the company says is - perspectives on European politics and policy.
He will not be stepping down as an MP though, and will have to declare this new role publicly.
Former chancellor George Osborne is to join the world's largest fund manager, BlackRock, as a part-time adviser.