The firm announced Saturday that Geithner will serve as its president and managing director starting March 1.
Geithner led the Federal Reserve Bank of New York for six years before being becoming Treasury secretary in 2009. By that time, the economy had sunk into a deep recession.
Few Treasury secretaries received as much scrutiny. Supporters credited Geithner with helping prevent the recession from spiraling into a second Great Depression, but critics said he was too cozy with Wall Street.
Geithner stepped down from Treasury in late January, days after President Barack Obama was sworn in for a second term, and signed a deal with Random House's Crown Publishers to write a behind-the-scenes book about the response to the economic crisis.
Geithner was the last of Obama's original economic advisers to leave the administration. He was succeeded as Treasury secretary by Jack Lew.