Pol Lt Gen Anuchai said Jom Imkaew, 63, was number nine on the wanted list with a 100,00 baht bounty on his head.
The suspect nonchalantly confessed that he killed at least 70 people when he was 21 to 50 years old, and that most of them were criminals or drug dealers. He was paid about 4,000 to 80,000 baht for each hit.
In 2006, he and a man known only as Sgt San shot dead a married couple who owned a noodle place in Chiang Mai's Chiang Dao district. He had been in hiding in Khon Kaen since then, he said.
According to the suspected assassin, he was sentenced to life in prison for killing a village chief in Chiang Mai's Fang district in 1980. But he was released in 1992 after receiving a pardon.
The man was convicted again in 1994 for a firearm-related offence and completed his jail time in 2006.