The U.S. Customhouse is a historic custom house located in San Francisco, California. It was built to house offices of the United States Customs Service. Possessing an extraordinary natural harbor and one of the country's finest ports, San Francisco rapidly expanded during the nineteenth century. By the turn of the twentieth century, construction of the Panama Canal, which would dramatically shorten trade routes between the Atlantic and Pacific, had begun. City officials likely anticipated increased Commerce and determined that a larger custom house was needed.