Bitcoin is legal -- or perhaps more accurately, not explicitly illegal -- though you can run into technical violation of the law using bitcoin in certain ways. For instance, if you're mining bitcoin, when you swap for US dollars on an exchange, those proceeds are technically taxable, though it's unlikely any tax authority will ever really care unless we're talking considerably sums of money. There are of course lots of these de minimis violations that it's rare to get into any kind of hot water for, for instance when you buy something from the internet and not paying sales tax, in most (and likely all, but I can't say for sure) states you're supposed to be voluntarily reporting those purchases and paying the sales tax yourself on your next state tax filing.