From the US embassy website:

ENTRY / EXIT REQUIREMENTS: All U.S. citizens must have a U.S. passport valid for at least six months from the date of arrival in Belize and a proof of return to the U.S. No visas are required for citizens of the United States for tourist visits of up to 30 days, but they must have onward or return air tickets and proof of sufficient funds to maintain themselves in Belize. Visitors for purposes other than tourism, or who wish to stay longer than 30 days, must obtain visas. All tourists and non-Belizean citizens are required to pay an exit fee of U.S. $39.25 (payable in U.S. dollars only). Belize allows visitors a maximum of one month’s stay in the country before they require an extension. No specific immunizations are required for visitors to Belize. Visit the Embassy of Belize website for the most current visa information.