This Beaux-Arts federal building, which borrows from McKim, Mead and White's Boston Public Library, has a smooth limestone foundation and terracotta skin covering reinforced concrete and hollow tile, under a red Spanish tile roof. The south front is typical of late Italian Renaissance Revival structures, as are the rich facade details in terracotta. The fanlight arch of the front entry, capped by a terracotta keystone, is echoed in six multipaned front windows, which are supported by a set of balusters and separated with pilasters and medallions under a wide, dentiled cornice. Large corner windows have molded sills supported by corbels and triangular pedimented heads. Oversized iron lanterns flank the main entrance in a rigidly symmetrical facade. The richly detailed lobby has Neoclassical ceiling molding, oak paneling, and marble bordering the terrazzo floor, with marble stairs to a small second floor that once housed offices. This late example of Beaux-Arts Neoclassicism is a throwback to an earlier era of monumental proportions, excellent detailing, fine materials, and high levels of craftsmanship. When the post office moved elsewhere in 1990, this little-altered landmark became the Fremont Center for the Arts.