United States passports are issued to citizens and non-citizen nationals of the United States of America. They are issued exclusively by the U.S. Department of State. It is unlawful for US Citizens to enter or exit the United States without a valid US Passport.
By Law, a valid unexpired US Passport also serves as a proof of US Citizenship, and has the same effect as proof of United States citizenship as Certificate of Naturalization. As a US Citizen, US law does not prohibit US citizens from holding passports of other countries, though they are required to use their US passports to enter or exit the United States.