A Singapore court rejected on Tuesday an appeal against the execution of a Malaysian convicted of drug smuggling, dismissing an argument put forward by his legal team that he should be spared because he was mentally impaired.

Nagaenthran Dharmalingam has been on death row for more than a decade for trafficking about 42.7g of heroin into Singapore, which has some of the world’s toughest narcotics laws.

He had appealed on the grounds of...