A Bangkok court heard that Somchai Keawbangyang suffocated the bedridden Japanese victim, Yoshinori Shimato, 79, and then discarded pieces of his body in plastic bags in a canal in Samut Prakan province, which adjoins the Thai capital.
Somchai, 48, confessed to the murder, which prosecutors said took place between mid-September and October 13 last year.
He was initially given the death penalty, but this was reduced to life imprisonment due to his confession.
She denied being an accomplice to the murder, but was given a 20-year sentence for her part in the crime.
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"Somchai killed Shimato due to his jealousy and it is believed that he dismembered his body after his death," the court verdict said.
"But there was no witness who saw Pornchanok colluding in the murder so the court gave her the benefit of the doubt," it added.
Shimato was in Thailand as a Japanese language teacher.
Swathes of Bangkok are dedicated to Japanese restaurants and karaoke bars specifically targeting expats from Japan.