The short answer is yes.
Americans, unlike citizens of 99% of the nations around the globe, are required to pay taxes on worldwide income. This means that no matter where you earn money, whether it’s from an employer’s paycheck, interest from a Swiss bank account, profits from foreign real estate, proceeds from a yard sale, or online gambling winnings, you owe Uncle his piece of the pie.
This is true regardless of whether or not said income is reported to the Internal Revenue Service by a third party. It's an honor system, to a certain extent, and the odds of getting caught are relatively small, though you can be sure the IRS can detect unreported income. However, if you do get caught, you can face severe civil and/or criminal penalties.
