Kho Jabing, 31, was sentenced to death in 2010 for killing a Chinese construction worker in a robbery gone wrong and spent the next six years on a legal roller-coaster trying to avoid the gallows.
A High Court judge in 2013 commuted his sentence to life imprisonment and caning following changes to the penal code that ended the mandatory execution of murder convicts.
But state prosecutors fought the decision and the Court of Appeal, Singapore's highest court, reinstated his death sentence in January 2015.
Judge Chao Tick Hin, who delivered the final decision of the five-judge court, said the motion did not introduce any new material compelling enough for the court to reconsider the death sentence.
"It is the applicant's core case that our decision in the re-sentencing appeal is wrong," the judge said, adding that material presented fell short of showing that the court made a mistake.
He said a new execution date would be set by the Singapore president, who has already rejected clemency.
"We are... Concerned that (Kho) has been forced to endure years of immense suffering as his sentence has been changed on a number of occasions," said Laurent Meillan, the UN Human Rights Office's regional representative, in a statement.
Human Rights Watch also expressed its opposition.
"For Singapore to defend the death penalty on international forums is a further indication of complete disregard for international human rights standards," Phil Robertson, deputy director of its Asia division, told AFP.
Singapore executed four people in 2015 -- one for murder and three for drug offences.
Malaysia also executes murderers and drug traffickers by hanging, a punishment which dates back to British colonial rule in both countries. But it does not publicly announce them.
Human rights groups have called on Singapore to abolish capital punishment but the government has rejected such calls, arguing death sentences must remain as a deterrent.
