The Game -- I'm a Pot-Smokin' Tax Fraud!

Writing off medical marijuana is totally legit ... so claims The Game -- problem is he's wrong ... and the IRS says its AGAINST THE LAW!

Game was at the Nokia Theater Saturday night in L.A. when he schooled our photog about writing off medical sticky icky.

Here's the rub ... according to the IRS, "You cannot include in medical expenses amounts you pay for controlled substances (such as marijuana, laetrile, etc.). Such substances may be legalized by state law. However, they are in violation of federal law and cannot be included in medical expenses."