Sycamore Partners will buy Staples for $10.25 per share, in a deal which values the office supplier at around $6.9 billion.
Staples' stock surged more than 8 percent into the close Wednesday, leaving its share price at $9.93.
Staples shares were halted during extended trading after rising an additional 1.5 percent following the close.
Sycamore has several holdings in retail, owning Talbots, Dollar Express, Coldwater Creek, Hot Topic and Nine West.
"We have tremendous confidence in CEO Shira Goodman and great respect for the Staples management team and are excited about this opportunity to partner with them," Stefan Kaluzny, Managing Director of Sycamore Partners, said in a statement.
The acquisition comes a year after a U.S. federal judge thwarted a merger between Staples and peer Office Depot Inc. on antitrust grounds.
It represents a bet by Sycamore that Staples could more quickly shift its business model from serving consumers to catering to companies if it were to go private.
Staples has 1,255 stores in the United States and 304 in Canada. It has the largest market share of office supply stores in the United States at 48 percent, and its share has increased since 2011, according to Euromonitor.
Private-equity acquisitions of retailers have become increasingly rare, as the investment firms worry about increasing headwinds facing the industry and their portfolio companies struggle with the debt burden left behind from leveraged buyouts. Retail deals comprised the smallest share of mergers and acquisitions in the first quarter of the year, according to Thomson Reuters data.
A number of private equity-backed retailers, from Sports Authority Inc. to Payless ShoeSource Inc, have filed for bankruptcy in the last two years.
Sycamore, however, specializes in retail investments and has been more bullish on the sector. Its previous investments include regional department store operator Belk Inc., discount general merchandise retailer Dollar Express and mall and web-based specialty retailer Hot Topic.
Reuters contributed to this report.