Rapper Ty Dolla $ign got popped for marijuana when Wisconsin police busted his tour bus -- but that's news to him ... since Ty insists he wasn't even on the bus.
According to Blue Mounds police chief Andrew Rose ... Ty's bus slowed down early Wednesday morning for equipment failure. Cops pulled over the bus to make sure everything was cool ... but officers say they were quickly blown away by the smell of pot.
Cops say they found about an ounce and a half of weed, and cited 7 people -- including Ty -- for possession and/or paraphernalia. But a rep claims Ty was on a plane to Omaha at that time, so there's no way he got a citation.
All cops know is ... they're expecting someone who goes by Ty Dolla $ign to pay the ticket -- which should be less than $500.