The currency of Sri Lanka is the Sri Lankan rupee (Rs or LKR). ATMs are common and usually offer the best exchange rate if you choose to be charged in rupees. Credit cards are also widely accepted, but you’ll need cash to pay in smaller towns and markets.
Currency should only be changed at banks or at official money changers. Avoid uncertified money traders on the street and try to get cash in smaller denominations as change can be hard to come by. You should re-convert all remaining rupees before leaving Sri Lanka.