Short answer: No.
Long answer: Well, no, but the size of your find is going to cause some legal concerns.
As a rule, any currency is technically a bearer bond, which means it belongs to the person who is holding the physical issue of currency at the moment of transaction, so if you find an amount of cash just lying around on the ground, and pick it up, you are legally the owner. This is why cash is preferred medium of exchange for your bad guys and there is a rumor that all U.S. currency will test positive when looking for a trace amount of a drug on the bill. Cash is difficult to trace by ordinary means.
Large sums may require you to report the find to police who have a window of time (usually a 30 day) window for someone to claim it. Like many legal elements in the U.S. this is based on your jurisdiction as to if there is a statutory period, or if merely showing an attempt to find the legal owner will count as protection against later charges of theft. Either way, once that matter of seeking but not finding the return is shown, it's legally not theft and legally yours. But we must discuss the elephant in the room here, namely, losing a million dollars in cash is actually pretty difficult.
First, despite what movies would have you believe, depending on the denomination of the bills found, a million dollars can weigh as little as 22 lbs (all $100 notes) to 1.1 tons (all $1 notes) and $1 million in the most commonly circulated note in USD ($20) is 110 lbs. Suffice to say, this is a significant and noticeable weight to just drop out of your pocket, but this is just the improbable situation, not anything problomatic.
What's more important is that per US law, banks must record all exchanges of currency to the value of $10,000 or more in a single transaction to federal law enforcement agencies with jurisdiction over money crimes (and don't get clever and move $9,999.99. Banks tend to catch on to these transactions and report them too, just to be safe). It actually takes a while as your standard bank may not even have that much physical cash on hand to minimize the loss from robberies. And often banks will note the serial numbers of some bills in the stacks, just in case that cash winds up in law enforcement hands... There's a very good way to at least track down the rich idiot who can lose $1 million in pocket change. If the money is aggregate, chances are it's related to several criminal transactions and might be siezed as evidence of a crime (and likely would be returned to circulation.
As for the IRS, while I don't know the specific box on the form to tick for found money of significant size, I do know there is a specifc box to check for money earned from criminal enterprise. Contrary to the popular assumption, Al Capone wasn't arrested for tax evasion because he wasn't paying his taxes, but because he was. He however was only reporting his legitimate earnings as taxible income, but clearly was living well above his means based on his reported income. And he wasn't shy about flashing his unreported wealth.
Heck, if you want to really screw the system, check your unlawful income and then claim a Ski-Mask and new super fast sports car as non-taxable business expenses (since they would have contributed to your illegal bank robbery which you no own taxes on). Suffice to say, the government doesn't much care how you got your money so long as it gets it's cut of the take. They'll have a box for ticking for found money.