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Quincy-based developer FoxRock is expanding its downtown Boston portfolio with the $43.5 million acquisition of a property in the South Station submarket.

The 85,527-square-foot office and retail building at the corner of Federal, Purchase and High streets was completed in the early 20th century, according to assessor’s records. 

The seller, Federal Street 176 Holdings, acquired the property in 2012 for $20 million, according to Suffolk County Registry of Deeds records.

FoxRock owns more than 4 million square feet of commercial real estate, including the 40 Beach St. garage in Chinatown. Last month, Foxrock submitted plans to develop a 20-story hotel-apartment tower and office building in Quincy Center. The firm is also redeveloping the former Quincy Medical Center campus into a 465-unit apartment community known as Ashlar Park.