Constitution Day (also referred to as Citizenship Day) is a United States federal observance that commemorates the signing of the U.S. Constitution and honors those who have become U.S. citizens by coming of age or naturalization.

It is observed annually on September 17.

The delegates to the Constitutional Convention signed the final version of the United States Constitution on September 17, 1787. In 1911, Iowa became the first state to recognize the anniversary of this event as Constitution Day. In 1917, a committee formed by the Sons of the American Revolution started promoting this day.

The law establishing the present holiday was enacted in 2004. On this day, all federal agencies and publicly funded educational establishments are required to provide educational programming on the history of the American Constitution. When Constitution Day falls on a weekend or another holiday, its observation is moved to the closest weekday.