In March, Geithner will become president and managing director of the New York City-based firm, CNNMoney reported.
Geithner, 52, told The Washington Post that he will play a "substantive role in helping ... manage the firm."
Warburg Pincus put out a statement saying Geithner will "work closely" with the firm's co-chief executives on strategy, management and investing.
Geithner, who began working for the Treasury in 1988, was most known for his TARP bank rescue plan, which was pushed through Congress by former Treasury Secretary Henry Paulson. Many saw the plan as a bailout for rich bankers, while other credited it with stabilizing the economy