The Criminal Court has sentenced a woman to death over the 2015 murder of her elderly Japanese partner, a member of the prosecution said on Monday.

Poranee Napadol, who is in her 50s, was arrested in October 2015 after her brother-in-law Sampan Chamchaeng told police he had killed Kazuo Yoshioka, then 84, at her request in Ang Thong province.

She was initially acquitted by the Provincial Court in 2016 for lack of evidence. The Appeals Court upheld the initial verdict.

The prosecution appealed the ruling to the Supreme Court, arguing she was the mastermind behind the murder. The court handed down its verdict on Thursday last week.

"The court believes Poranee is the key person who commanded her brother-in-law to kill her own husband, despite her protestations of innocence from the beginning," the prosecutor said.

Yoshioka, a retired businessman from Japan's Hyogo prefecture, was on Oct 13, 2015, found with his throat slit at the house that he shared with Poranee.

Sampan, who is married to Poranee's younger sister, was in 2016 convicted of murder and sentenced to life imprisonment.