The United States Patent and Trademark Office (USPTO) was established under Article 1, Clause 8 of the United States Constitution:
"The Congress shall have Power To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries."
It is an agency under the U.S. Department of Commerce.
United States Patent and Trademark Office homepage (http://www.uspto.gov/)