One Post Office Square is a 42-floor modern skyscraper in the Financial District section of Boston, Massachusetts. The skyscraper is Boston's 14th-tallest building, standing 525 feet tall. The building has 831,975 square feet of Class A office space. An attached, eight-level parking garage offers 368 parking spaces and direct access to the building's lobby. It overlooks Post Office Square Park and was designed by Jung Brannen Associates. A previous building on the site at 101 Milk Street was the headquarters of the Boston Elevated Railway until about 1919.