Jim Belushi -- Weed SNAFU in Martha's Vineyard

Jim Belushi just learned a valuable lesson -- don't fly into Martha's Vineyard with a fat ass joint in your pocket ... unless you wanna lose it.

Sources tell TMZ, Jim was at the uppity Martha's Vineyard airport on Friday -- when TSA agents discovered a marijuana cigarette on the actor's person.

We're told Jim tried to justify his pot by brandishing his California medical marijuana card -- but he quickly learned ... medical marijuana cards don't mean jack in Massachusetts.

We're told the West Tisbury Police were notified ... but since the amount was so small -- and marijuana possession has been decriminalized in Massachusetts -- cops just confiscated the drug and let Jim off with a warning.

Total buzz kill. Calls to Jim's camp were not returned.