The Constitution is the law of the United States. 
https://constitution.congress.gov/constitution/ 

There are 7 articles to the Constitution and over the years, things have had to be added to it and changed, due to the change in times. These changes are called Amendments and there are 27 amendments to the US Constitution. The constitution was signed into law September 17, 1787. Each year on September 17th, it is declared to be Constitution Day and the week of September 17-23 is Constitution Week. August 2, 1956 the US Congress designated September 17-September 23 to be Constitution Week and President Dwight D. Eisenhower signed it into law. September 15, 2017 President Trump proclaimed Constitution Day, Citizenship Day and Constitution Week.