All arriving visitors to the DR need a valid passport for entry. Your passport must be valid for at least six months after the date of return, and it must have sufficient pages for entry and exit stamps.
***AS OF APRIL 2018, tourists who previously needed to purchase a US$10 tourist card ("Tarjeta de Turista") upon entry NO LONGER need to do so as the US$10 fee is now included in the airfare. Check out this blog post for more information.
Tourist visits are valid for 30 days. Should you overstay by a few weeks or months, you will bear a scaled exit charge at the airport when you leave. The current charges range from US$60 (RD$2,500) for 1 to 3 months overstay to US$115 (or RD$5,000) for one year.
The customs lines can be a pain or it can be easy, depending on the airport in which you land. They usually don’t go through your things if you are traveling light and are a short-term tourist.
ALWAYS consult your Embassy to determine if you need to obtain a separate visa for entry to the DR.
For Americans staying a week or more, it's always a good idea to register your presence in the DR with State Department's Smart Traveler Enrollment Program (STEP)–for keeping up to date on any important travel alerts and news.
Partly excerpted from Moon Dominican Republic (Moon Travel Guides, 2016 edition) - to be updated this Fall.