From the NHL website:
Union Oyster House is both the oldest continuously operated restaurant and the oldest continuously operated oyster bar in the United States. The establishment has hosted numerous famous patrons and is a monument to the importance of oysters in American culture during the 19th century. Ongoing architectural research is augmenting our appreciation for the architectural significance of the building as a rare survival of early 18th-century Boston.
The Union Oyster House is on the National Register #03000645, and it is also in the Blackstone Block Historic District, on the National Register #73000315. The Union Oyster House is also a National Historic Landmark.