A slippery passenger was busted trying to get a free ride on a Brooklyn bus, but it wasn’t your typical fare-beating punk — it was a boa constrictor.

The scaly stowaway slipped along a back window of the B12 bus as the driver was heading along East New York Avenue on Saturday afternoon.

After spotting the large snake, the driver quickly brought the bus back to the depot, where workers had to pull off panels to get to the creature, which appeared to be headed to the warmth of the engine. It was taken away by workers from Animal Care and Control.

The bus was eight stops into the route when the driver noticed the snake, but the MTA insisted there were no riders aboard.

An MTA source said another driver had seen a passenger bring a snake onto the bus in a bag the day before.

When the man got off, the driver assumed he took the snake with him — until it turned up the next day.

“A guy brought a snake with him on the bus,” the source said. “I guess it got too big for him to take care of, so he left it on the bus in a bag.”

Regular riders on the B12 route were shocked.

“Man, that is one big snake,” said Miles Rivers, 42. “How did that thing get on the 12?”

Another incredulous rider said the find could provide her with a new fashion accessory.

“A snake, really?” she said. “I hope it’s big enough because I need new shoes and a bag.”

Others compared the reptile to characters regularly seen riding public transit.

“I have seen plenty of two-legged snakes on the bus, but never one like that,” Sienna Ortiz, 23, marveled. “It’s huge!”

A similar incident took place in August 2013, when subway riders were shocked to find a dead shark riding the N train.

A conductor reported that there was “a shark aboard the train” and emptied the car.

A service supervisor reported that the shark — a female smooth dogfish — was dead and tossed it in the trash at the Ditmars Boulevard station in Astoria, Queens.

“This is New York City,” one straphanger said at the time. “What could surprise you here?”

Additional reporting by Dana Sauchelli