Generally not required for under 90 days. Belize issues 30-day visas. The Centro America 4 (CA-4) agreement allows 90 days in Guatemala, Nicaragua, Honduras and El Salvador.
At present citizens of the USA, EU, Canada, Australia, New Zealand and many other nations can arrive in all Central American countries (including Mexico) without arranging a visa beforehand. Check ahead from your country before planning your trip, as this may change.
Many countries charge an entry or tourist fee upon arrival – from US$5 to US$20.
Note that, if you need a visa for a certain country and arrive at a land border without one, you will probably have to return to the nearest town that has a consulate and obtain a visa. Airlines will not normally let you board a plane to a country for which you don’t have the necessary visa. Also, a visa in itself may not guarantee entry: in rare cases, you may still be turned back at the border if you don’t have sufficient funds for your visit or an onward or return ticket.
Having your passport checked is a routine procedure upon arrival in a country, but some officials may ask about your financial resources, either verbally or on the application form. If you lack ‘sufficient funds’ for your proposed visit, officials may limit the length of your stay. (US$500 per month for your planned stay is generally considered sufficient; traveler’s checks, and sometimes a credit card, should qualify toward the total amount.)
Several Central American countries require you to have an onward ticket leaving the country.
Once you are inside a country, you can always apply for a visa extension at the country’s immigration office (migración). There's usually a limit to how many extensions you can receive; if you leave the country and reenter, your time starts over again.
Guatemala, Honduras, El Salvador and Nicaragua’s ‘CA-4 Border Control’ agreement allows free travel for up to 90 days within this sub-region for citizens of the four countries and many foreign nationals (including residents of the USA, Canada, the UK and Australia).
On paper, at least, you should only have to pay a tourist fee once to enter these four countries. Yet border patrols may also charge you a few dollars for ‘paperwork’; if they insist, you won't have much alternative but to pay.
Make sure your passport is valid for at least six months past your arrival date and has plenty of blank pages for stamp-happy officials. Always keep it with you while traveling between destinations.
All visitors leaving and entering a Central American country go through customs. Be prepared for bag checks at both airports and land borders. Most are just a quick gaze-and-poke, more of a formality than a search – but not always. Be polite to officials at all times.