SL Green Realty Corp., Manhattan’s biggest office landlord, agreed to buy the HarperCollins Building on East 53rd Street in Midtown for about $265 million, two people with knowledge of the deal said.

The price values the 37-story tower at about $680 a square foot. The price for the 388,000-square-foot tower at 10 E. 53rd St. was kept down by the expiration of leases for about 80 percent of the building in 2014, according to one of the people. At least $30 million of renovations will be needed to re-lease the space, the person said. SL Green declined to comment.

Among the tenants with a lease expiring is HarperCollins Publishers, owned by News Corp, which also owns The Post.

The deal caps a year in which SL Green was the biggest buyer of New York commercial property, according to data from Real Capital Analytics Inc.