Yes, whether you are traveling from the United States or any other country from around the world a valid passport is required for entering Santo Domingo. If you are traveling from the US a valid US passport is required, with at least one blank page for an entry stamp. The US citizens also don't require a visa for up to 30 days when visiting the Caribbean nation. In lieu of a visa, a 30-day tourist card needs to be purchased ($10) at customs and immigration desk or online before you fly to Santo Domingo. You will also have to pay a $20 departure tax, which is typically included in the cost of your airfare. You should however make sure to check with the airline you will be flying.