The Old Customshouse is a historic government building at 516 North King Street in Wilmington, Delaware. It was built in 1855 and added to the National Register of Historic Places in 1974.

Ammi Burnham Young was a 19th-century American architect whose commissions transitioned from the Greek Revival to the Neo-Renaissance styles. His design of the second Vermont State House brought him fame and success, which eventually led him to become the first Supervising Architect of the U.S. Treasury Department. As federal architect, he was responsible for creating across the United States numerous custom houses, post offices, courthouses and hospitals, many of which are today on the National Register. His traditional architectural forms lent a sense of grandeur and permanence to the new country's institutions and communities. Young pioneered the use of iron in construction.