Chrishan Beasley, 32, sawed through steel bars above his cell with a hacksaw and used a rope to rappel down a 20-foot wall, authorities said.
The Californian was arrested in August at a post office, carrying a package containing 5.7 grams of hashish, The Associated Press reported.
He escaped with another U.S. citizen, 57-year-old Paul Anthony Hoffman, who was serving a 20-month sentence for robbery. Hoffman was captured as he climbed down the wall. Beasley, who had gone over first, managed to run away.
Kerobokan is a hellish facility known for brutal violence, overcrowding and too few guards.
An investigation is underway to determine if prison staff helped the men, according to a spokesperson for the Law and Human Rights Ministry.
The prison on Indonesia’s resort island was designed to hold 300 people, but has nearly 1,600 inmates.
The country has extremely strict drug laws that sometimes include the death penalty. Beasley was awaiting sentencing for his conviction.
It was the second time since June that foreign prisoners have escaped the institution. Four inmates fled through a drainage tunnel. Bulgarian Dimitar Nikolov Iliev and Indian Sayed Mohammed Said were arrested days later in East Timor.
Shaun Edward Davidson of Australia and Tee Koko King bin Tee Kim Sai of Malaysia are still at large, the wire service reported.