A Thai gunman was sentenced to life in prison on Friday for the murder of a Cambodian opposition politician in Bangkok, a judge said.

French national Lim Kimya, a former opposition lawmaker in Cambodia, was shot dead on January 7 by Ekkalak Paenoi as the ex-MP visited Bangkok with his wife.

Cambodian opposition figures have accused the country's powerful former leader Hun Sen of ordering the shooting, and Lim Kimya's widow called this week for a full accounting of who was behind it.

"The actions of the first defendant caused harm to the plaintiff," the Bangkok Criminal Court judge said on Friday.

"Since he confessed, the court reduced the sentence to life imprisonment."

Ekkalak was arrested in Cambodia a day after the shooting and confessed to committing the murder in a livestream video.

The trial began just three days ago with an examination of witnesses, including Lim Kimya's widow, Anne-Marie Lim.

"Anne-Marie is probably satisfied with today's verdict, but she is still questioning who ordered the crime," her lawyer Nadhthasiri Bergman said outside court.

"She wants authorities to get to the bottom of it."

"We know the suspects are in Cambodia, and [the Thai government] could help push the extradition process to bring them to justice," Bergman added.

The judge did not offer details about the killer's motive or a possible mastermind behind the murder.

Thai police said in January that they were also seeking to arrest a Cambodian national believed to be behind the killing.

The court on Friday dismissed the charges against a second defendant, Thai national Chakrit Buakhil, who was accused of driving Ekkalak to the Cambodian border after the shooting.

"He was only a driver and did not know what was happening," Chakrit's lawyer Natchapong Moosakopas said.

Some Thai media outlets reported this year that Ekkalak was paid 60,000 baht, or HK$14,040, for the killing, but police said he claimed he did not receive payment and took the job "to pay a debt of gratitude".

Cambodia's Prime Minister Hun Manet has denied his government or his father Hun Sen's involvement in the murder.

The former premier led Cambodia for nearly four decades until 2023, and Western nations and rights groups have long accused his government of using the legal system to crush the opposition. (AFP)