Officers were called to the pool at about 6pm and the youngster was later taken unconscious to Singapore General Hospital where he was pronounced dead.
According to Chinese evening daily Lianhe Wanbao, the boy was spotted playing with a friend at the deeper end of the pool, where the water reaches about 1.2m.
A guest staying at a nearby hotel told The Straits Times that “a large crowd had gathered” when the youngster was pulled out and two lifeguards attended to his motionless body before medics arrived.
A woman – believed to be in her 50s and related to the youngster – appeared distraught and had to be placed in a wheelchair and pushed away.
A Resorts World Sentosa spokesman said that the company “is deeply saddened by the incident” and is assisting with investigations.
He added: “Our lifeguards on duty immediately performed CPR for the boy and the paramedics, upon arrival, continued to administer CPR while the ambulance was activated. The team also provided assistance to the family on-site.”
Police are treating the incident as an unnatural death.