St. Kitts and Nevis use the Eastern Caribbean dollar. That currency is tied to the US dollar, with 2.7 EC dollars always equaling 1 dollar US . If you do wish to obtain some local currency you can exchange your own currency in Basseterre or Charlestown at one of the banks.

If you intend to ride the local buses around the island, it is useful to get some Eastern Caribbean currency (coins preferably) but generally speaking, if you have US dollars, almost every other business is happy to take those instead of local currency.

Note: if you are about to take a taxi, ask first how much it will cost, and also make clear whether the price quoted to you is in dollars US or dollars EC -- there is a big difference!