Theerasak Longji was executed on Monday, six years after his conviction.
His death came as Thailand's coup leader-turned-premier Prayut Chan-O-Cha prepares to travel to Britain and France on a highly-publicised official visit.
But the trip will now likely see the former army chief face awkward questions over the use of the death penalty as well as Thailand's wider human rights record since he seized power in a 2014 coup.
"We still have the death sentence, we have not cancelled it yet," Tawatchai Thaikaew, deputy permanent secretary at the Justice Ministry, told AFP, adding that the execution on Monday was carried out "according to the law".
Thailand's Department of Corrections, which oversees one of the world's largest prison populations, said 325 convicts have been executed since 1935, the majority by firing squad.
That practice ended on December 11, 2003. Between then and 2009 a further six inmates were executed by lethal injection.
Monday's execution serves as a "lesson to deter those who wanted to commit serious crime" the department added in a statement.
But rights groups hit out at the sudden resumption of the death penalty, which remains mandatory for a number of offences, including aggravated murder.
"This is a deplorable violation of the right to life," Amnesty International said, accusing the kingdom of "reneging" on commitments to move towards abolition of the death penalty.
Thailand was "also putting itself out of step with the current global shift away from capital punishment".
Theerasak stabbed his victim 24 times before stealing their mobile phone and wallet, the department said.
Nearly 200 had exhausted all final appeals -- like Theerasak.
More than half are believed to have been sentenced for drug-related offences.
The death penalty is still in practice in several countries in Asia, including Singapore, Indonesia and China, which remains the world's top executioner.
The International Federation of Human Rights also condemned the move in Thailand, calling it a "betrayal".
It said the country would have achieved the status of "de facto abolitionist" had it not carried out any executions before August 24, 2019, 10 years after the last death sentences were carried out.
Thailand's justice system has been criticised for favouring the wealthy and connected and for falling short in several high-profile cases that have made headlines in recent years.
In 2015 two Myanmar migrant workers were sentenced to death for the 2014 murder of two British backpackers, one of whom was raped, on the diving island of Koh Tao after an investigation and trial that was widely considered flawed.
Their lawyer told AFP on Tuesday that they were waiting for a ruling on their final appeal.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)
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