Bring either a U.S. passport or a certified copy of a birth certificate and photo identification.
Entry Requirements
Proof of citizenship and photo identification are required for entry by all U.S. citizens. A passport and visa are not required for a tourist/transit stay of up to 180 days. A tourist card, issued by Mexican consulates and most airlines serving Mexico, is required. Minors require notarized consent from parent(s) if traveling alone, with one parent, or in someone else's custody.
Returning to the United States – Caution! Make certain that you can return to the United States with the proof of citizenship that you take with you. Although some countries may allow you to enter with only a birth certificate, U.S. law requires that you document both your U.S. citizenship and identity when you reenter the United States.
The best document to prove your U.S. citizenship is a valid U.S. passport. Other documents that establish U.S. citizenship include an expired U.S. passport, a certified copy of your birth certificate, a Certificate of Naturalization, a Certificate of Citizenship, or a Report of Birth Abroad of a U.S. citizen. To prove your identity, either a valid driver’s license or a government identification card that includes a photo or physical description is acceptable.