The suspect escaped to hide in Thailand. He is wanted in Cambodia, where he has a long criminal record.
The Man, identified as Sok Na, 28, together with four companions, killed Kitakura Kosei, 44, a Japanese businessman based in Phnom Penh, on March 3. They took US$18,000 (B485,000) cash.
The gang had been observing the casino in Poi Pet. Once they saw Mr Kitakura win at gambling, they signaled one another and lured the victim into using their bicycle rickshaw service.
The Japanese man was shot dead with a 9mm gun while on the road, and the suspects escaped separate ways.
Cambodian authorities had earlier arrested three members of the group.
Sok and another gang member received about US$400 each (B10,000) to spend during the Songkran holiday in Thailand. Sok then left to work at a shop in Thailand before being arrested by the Thai authorities, while the other man is believed to be still hiding in eastern Thailand.
Cambodian police are seeking Sok Na in relation to two murder cases last year, the first involving robbing and killing a Khmer student and the second involving the death of a Chinese tourist.