The currency of Bhutan is the ngultrum (Nu or BTN), the value of which is tied to the Indian rupee. Cash is still the preferred method of payment here, although credit cards are accepted in larger hotels and stores, but you may be charged a fee.
You’ll find ATMs in Thimphu, Phuntsholing, Paro, and Punakha, but it’s a good idea to bring some cash, ideally US dollars in smaller denominations.