Dennis put $2 in the register and stuffed the rest in her bra, Swissvale Police Chief Greg Geppert said.
After the arrest, cops found Dennis was holding 50 more bags of heroin, along with a small amount of pot.
She was booked on charges of possession, delivery and possession with intent to deliver, the station reported.
Investigators aren’t sure how much heroin was sold at the McDonald’s before Wednesday’s bus.
Earlier this month, another McDonald’s employee was busted for selling smack out of a restaurant in Murrysville, about 20 minutes outside of Pittsburgh.
Neither case appeared to be related to the pain killer-spiked heroin that has killed 22 people in southwestern Pennsylvania in recent weeks.