Osborne will work for the BlackRock Investment Institute (BII), an arm of the firm that brings together and shares investment perspectives for BlackRock investors and clients.
He will take up the position on 1 February.
Osborne resigned as chancellor of the exchequer shortly after the EU referendum in June, but remains a member of Parliament. He held the role of chancellor from May 2010 to July 2016.
In his role as senior advisor, BlackRock said the former chancellor "will provide perspectives on European politics and policy, Chinese economic reform, and trends such as low yields and longevity and their impact on retirement planning".
It added that Osborne will not engage in any lobbying of the British government, in accordance with UK law.
Laurence D Fink, chairman and CEO of BlackRock said, "George has a unique and invaluable perspective on the issues that are shaping our world today.
"At the centre of our mission is helping people around the world save and invest for retirement, and George’s insights will help our clients achieve their goals."
Osborne commented, "I am excited to be working with the BlackRock Investment Institute as an advisor.
"BlackRock wants better outcomes for pensioners and savers - and I want to help them deliver that. It's a chance for me to work part-time with one of the world's most respected firms and a major employer in Britain."