The arrest, all caught on video, shows the councilman asleep with a crack pipe before officers wake him.
A concerned passerby had seen Matthew Reilly and called police.
Police also found a bag of cocaine at the scene.
Even officers seemed to be shocked by what they found.
"You're a councilman in Cranston, smoking crack, with crack on you. What do you think the constituents would say?" An officer asked Reilly, bodycam footage shows.
Reilly told them he had smoked crack earlier at his home after being clean for 13 years.
He says he relapsed due to a bad divorce.
Reilly, who is an attorney, also resigned as chairman of the Cranston Republican Party.