Thailand's Supreme Court has rejected the final appeal of a former headmaster sentenced to death for the murder of three people, including a toddler, in an armed robbery in 2020, officials said on Thursday.

The two-year-old boy was outside a jewellery store in the central city of Lopburi when Prasittichai Khaokaew burst in, firing indiscriminately, in January 2020.

A security guard and a sales assistant were also killed and four more people wounded in the robbery, which saw the ex-primary school head flee with jewels worth $16,500.

He was eventually apprehended by commandos following a manhunt.

"His behaviour was cruel and inhuman, unlike his occupation as a school headmaster," the court said in a statement released on Thursday. "There is not enough reason to reduce the penalty."

The court said Prasittichai had behaved "outrageously" and confirmed the death sentence imposed by the kingdom's lower courts.

"There is no reason that the Supreme Court would change that," it said.

Thailand has 250 prisoners on death row, according to the Department of Corrections, but executions are rarely carried out.

The last prisoner put to death was a 26-year-old murderer executed by lethal injection in 2018.

Most death sentences are commuted to life imprisonment, either by the courts or by the king as an act of royal clemency.

Human Rights Watch Asia division deputy director Phil Robertson urged the kingdom to downgrade the man's sentence to life in prison.

"Thailand should abolish the death penalty once and for all if it truly wants to be considered a rights-respecting state," he said. (AFP)