Former U.S. Treasury Secretary Timothy Geithner, one of the architects of the federal government's rescue of the financial system, is joining private-equity firm Warburg Pincus LLC.

Mr. Geithner, who has spent most of his career outside the private sector, said in an interview he plans to start in March at the New York-based firm, known for its role in buyouts of companies including eye-care firm Bausch & Lomb Inc., luxury retailer Neiman Marcus Group Inc. and stadium concessionaire Aramark Corp.

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