Helsinki is the capital of Finland, and Finland is a member of the European Union. This means that they use the euro as currency in Helsinki. But note that Finland is largely a cashless society these days, so you won't find much need for cash (if any). Be prepared to use tap-to-pay in most places in Helsinki. Be sure to travel with a credit card with no foreign transaction fees, and opt to be charged in euros instead of USD (or your home currency) if you're ever asked. (This will get you the best exchange rates.) Or have your Apple Pay/Google Pay set up to use contactless..