No worries about changing money — the US dollar is Cambodia’s unofficial second currency.
The official currency in Cambodia, the Cambodian riel, trades at around 4,000 riels to the US dollar. But there’s a 90 percent level of dollarization in the country. What this means is that you don’t need to be concerned about getting riel when you arrive in Cambodia. The visa you get on arrival must be paid for in US dollars.
Most tourist-oriented businesses quote prices in dollars, as do most grocery stores in the larger cities. Small stores may quote prices in riel, but they will all accept dollars as payment. Unlike in many countries, if you pay in US dollars, the exchange rate you will get is quite fair. At the time of writing the official exchange rate is 4,025 riel to the dollar, versus the street rate of 4,000 to the dollar, a difference of about 6 cents.
At border towns, such as Koh Kong or Ha Tien, you’ll often find the currency of the neighboring country also in use, meaning there can be three currencies in circulation: the riel, the dollar, and the Thai baht or the Vietnamese Dong. If you go through a land border, you may be told that there are no ATMs in the next town, or that you need Cambodian Riel to get through. This is not true, and visa prices are charged in US dollars.