The Internal Revenue Service doesn't care how or where you get your money they just want you to declare it. However, if online casinos don't declare your winnings then why should you? In the US real casinos declare the major winnings of their players. Not only that, but if you are a non-resident alien they also will take the taxes out before you even receive your winnings (on all games except blackjack, baccarat, craps, roulette and big-6 wheel). Sharing 30% of your $1500 Keno win can be a tad annoying, especially if you wagered a lot before you won.
In the US gambling wins are taxed at the highest rate allowed. That means you will be sharing up to 50% of your wins with the federal government, if you declare your winnings. So why declare? US citizens are required by law to declare all significant (above certain amounts, depending on the game) winnings to the IRS even when offset by losses. Not declaring and getting caught could mean big trouble. You should declare regardless of the fact that online casinos are off-shore and probably aren't declaring your winnings either. US federal government may not know about that $2,000 you won last week in the online Blackjack tournament. However, if you are audited you still need to explain where the money came from. At the end of the day, our advice is to follow the laws of your country and not fool with the IRS.
If you are a high roller and decide to declare, winnings are reported as "other income" on US tax returns. You only have to report the net gain on the wager. In other words, if you won $2000 on a $200 wager in Blackjack you only need to declare $1800. Unfortunately you can not offset your total losses against that win, so tough luck if you went on to wager and lose that $1800 later (this is why it’s so important to cash out when you win!).
Remember, you only have to declare the big wins. You can find out the exact amounts for the US at http://www.irs.gov. As mentioned above, you have to declare any wins of $1500 and above in keno, $1200 and above in slots and bingo. All other games with winnings of $600 and above must be declared.
Losses are listed as itemized deductions, not offset against wins, so you must keep track of them independently. Gambling losses are listed under "other miscellaneous deductions" and are fully deductible as itemized deductions. You can deduct up to the amount you declare in your winnings. You can not show a gambling tax loss. In other words, if you declare $1800 in your winnings you can declare up to, but no more than, $1800 in your itemized deductions under losses.
You must be able to prove your wins and losses, so keep your receipts, tickets, statements or other records that show the amount of both your winnings and losses. The IRS site also recommends that you keep a diary of your losses and winnings that includes: the date and type of specific wager, name and address of casino, anyone present with you while you were gaming, and the amounts won or lost. You should also keep a detailed book of your depositing and cash-out methods (ex. Credit card deposits and bank draft cash-outs)
In short, be mindful of all of your gambling records and keep them organized. So you shall be able to avoid the wrath of a vengeful Internal Revenue Service.