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 more than five years before becoming Treasury secretary in 2009, when 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 by stabilizing the banking system and restoring investor confidence. Critics said he was too cozy with Wall Street.
Warburg Pincus said that Geithner would advise the firm on strategy, investing, investor relations and other topics. The New York-based firm has been involved in buyouts of such well-known companies as luxury department store chain Neiman Marcus and contact lens maker Bausch + Lomb.
The firm declined to comment on Geithner's compensation. Through an aide, Geithner declined an interview request.
Geithner, 52, stepped down from Treasury in late January, days after President Barack Obama was sworn in for a second term. He was the last of Obama's original economic advisers to leave the administration, and was succeeded as Treasury secretary by Jack Lew.
In an interview with The Associated Press on his last day in office, Geithner said that the economy was "stronger than people appreciate" and predicted a pickup in growth. He defended his role in bailouts for large banks—steps designed to stabilize the financial system—but acknowledged that he would never win over his critics because it was hard to convince people about the dangers posed by the financial crisis.
The official who oversaw taxpayer bailouts of the banks, for example, criticized Geithner for allowing insurance giant American International Group to pay huge bonuses to executives. AIG got the biggest bailout of the financial crisis.
Geithner's appointment calendar from 2009 detailed his extensive contacts with CEOs of Goldman Sachs, JPMorgan Chase and Citigroup.
Since leaving office, Geithner signed a deal with Random House's Crown Publishers to write a behind-the-scenes book about the response to the economic crisis and has given speeches.
Geithner has spent most of his career in government, although he had an early stint at Kissinger Associates, the consulting firm formed by former Secretary of State Henry Kissinger. Geithner joined the Treasury Department in 1988 and served as undersecretary for international affairs during the Clinton administration. He worked at the International Monetary Fund from 2001 until 2003 before being named president of the New York Fed.
Private equity firms pool money from clients such as pension funds and other institutional investors to buy companies or stakes in companies. They try to improve the financial results of a company with the goal of reselling it at a profit.