BBC reports Tangaraju Suppiah, 46, was hanged at Changi Prison at dawn on Wednesday, his family said.
Activists said he had been convicted on weak evidence and had had limited legal access during his prosecution.
The authorities said he had received due process and criticised activists for questioning the courts.
Singapore has some of the world's toughest anti-drug laws, which it argues are a necessary deterrent to preventing drug crime.
