The prison's chief, Suraphol Kaewparadai, said yesterday that Chalermchai was granted parole because he had served more than two-thirds of his term and had demonstrated good behaviour.
Chalermchai was convicted of killing Preena Leepattanaphan, the governor of Yasothon, in 2001.
The murder attracted huge media attention. Based on solid evidence, the court initially handed down a life sentence. However, while behind bars, Chalermchai behaved well and received penalty reductions under royal mercy four times.
Through penalty reductions, he escaped the death penalty and saw his jail term cut to 17 years and 24 days.
Chalermchai will be on parole for nearly three years - not required to stay behind bars, but allowed to go out of jail for the remainder of his term on the condition that he report to probation officers occasionally and respects other conditions associated with parole.
"If he violates the probation rules or commits another crime during parole, he will be sent back to jail," Suraphol said.