U.S. citizens are required to have a passport that's valid for their entire stay in Belize. Citizens from the U.S., British Commonwealth, Finland, Greece, Denmark, Italy, Iceland, Leichtenstein, Tunisia, Turkey, Switzerland, Spain, Mexico, Uruguay, and Luxemburg don't need a visa. You're given a 30-day tourist pass upon arrival; if you want to stay longer than 30 days, you can visit an immigration office in Belize to get a new stamp or else cross the border and return.