Every traveler to Nicaragua  must have a passport valid for at least six months following the date of entry. A visa is required only for citizens of the following countries: Afghanistan, Albania, Bosnia-Herzegovina, Colombia, Cuba, Haiti, India, Iran, Iraq, Jordan, Lebanon, Libya, Nepal, Pakistan, People’s Republic of China, People’s Republic of Korea, Somalia, Sri Lanka, Vietnam, and Yugoslavia.
Everyone else is automatically given a tourist visa at the airport or land border, good for three months, in the form of a $5 tourist card. Don’t lose the card! You’ll need to return it when you leave the country. Technically, you must have an onward/return ticket, and evidence of sufficient funds; in practice, this is never checked.
Renewing a visa has gotten easier in the new tourist-friendly Nicaragua. In Managua  at the Metrocentro shopping mall, there’s an Immigration branch office (no phone, open 10 a.m.–6 p.m. Mon.–Fri., 10 a.m.–1 p.m. Sat. and Sun.) that will process your request for an extension. You’ll pay $0.25 for the form, and must present photocopies of your passport’s information page as well as the page with your visa.
A 30-day tourist visa extension is $10, and if you’ve overstayed your current visa you will pay an additional $1 for each day over the limit. Many travelers in San Juan del Sur  prefer to just take a bus over the Costa Rican border, and return the same day on a fresh, new tourist visa.
Furthermore, Nicaragua entered into the CA-4 Border Control Agreement permitting travelers to travel among Nicaragua , Guatemala , Honduras , and El Salvador without getting additional visas: Your initial entry visa is valid for the whole region for up to 90 days and can be extended once with little hassle.
Anything more serious than a basic tourist visa extension requires a trip to the main Office of Immigration (Dirección General de Migración y Extranjería, 1.5 blocks north of the semáforos Tenderí, tel. 505/2244-0741, 505/2244-1320, or 505/2244-3960, open 8:30 a.m.–noon and 2–4:30 p.m. Mon.–Fri.); show up at least four days before it expires with your passport, current visa, and $25.