A Bangkok court upheld the death penalty for Spaniard Artur Segarra on Friday after finding no substantial grounds for an appeal.
Segarra told EFE he would challenge the ruling at the Supreme Court.
The main defense for Segarra was that he denied all charges against him, his lawyer told EFE.
Segarra was sentenced to death for the murder of fellow Spaniard David Bernat in 2016.
While reading out the ruling, which lasted 10 minutes, the magistrate described Segarra's allegations as baseless and held as valid the evidence and testimonies presented by the police, which were accepted by a court during the trial in 2017 when the Spaniard was sentenced to death.
The convict, who was chained, remained serious during the hearing and demanded to be taken to a hospital afterward.
In a letter to EFE, Segarra had earlier maintained he was not guilty, and that he was a victim of a plan hatched by his Thai ex-girlfriend Pritsana Saen-ubon - who had testified against him in December - in connivance with the police.
A Bangkok court had handed down the death sentence to Segarra in April 2017, after finding him guilty of killing Bernat to access his bank accounts in Singapore.
Segarra - who has been kept in the high security prison in Klong Prem in northern Bangkok - was also found guilty of 12 other crimes, including kidnapping, torture and robbery.
He had absconded to Thailand from Spain, to escape conviction for his alleged role in a scam involving around 90 retired people.