A National Historic Landmark, it is an excellent example of Classical Revival architecture, and a rare surviving example of an early 19th-century public hospital building.

[2] The Bulfinch Building is a rectangular structure, two stories in height, with a massive Ionic portico at the center of its longer facade.

It has a hipped roof, and the central portion has a square attic story with chimneys at the corners and a saucer-shaped dome in the center.

[2] As designed by Charles Bulfinch in 1817 and built over the next five years by Alexander Parris, the building had smaller wings (roughly half the present size), and had a capacity of 73 beds.

The building's capacity was nearly doubled in 1844-46 by the addition of five bays to each of the wings, and the original entrance hall designed by Bulfinch was extensively altered.