A Thai woman was sentenced to death on Wednesday for poisoning a friend with cyanide, in the first of her 14 murder trials.

Sararat Rangsiwuthaporn, 36, an online gambling addict, is accused of swindling thousands of dollars from her victims before killing them with the chemical.

A court in Bangkok convicted her on Wednesday for fatally poisoning her friend Siriporn Kanwong.

The two met up near Bangkok in April last year to release fish into the Mae Klong river as part of a Buddhist ritual.

Siriporn collapsed and died shortly afterwards and investigators found traces of cyanide in her body.

Police were then able to link Sararat to previously unsolved cyanide poisonings going back as far as 2015, officers said.

"The court's decision is just," Siriporn's mother, Tongpin Kiatchanasiri, told reporters following the verdict.

"I want to tell my daughter that I miss her deeply, and justice has been done for her today."

Police said Sararat funded her gambling addiction by borrowing money from her victims -- in one case as much as 300,000 baht (nearly US$9,000) -- before killing them and stealing their jewellery and mobile phones.

She lured 15 people -- one of whom survived -- to take poisoned "herb capsules", they said.

Sararat faces 13 more separate murder trials, and has been charged with around 80 offences in total.

Her ex-husband -- a police lieutenant-colonel -- was given 16 months in prison and her former lawyer two years for complicity in Siriporn's killing, the lawyer for the victim's family said.

Thailand has been the scene of several sordid and high-profile criminal cases.

Earlier this year, six foreigners were found dead in a luxury Bangkok hotel after a cyanide poisoning believed to be connected to debts worth millions of baht. (AFP)