Natalie Cole's Son Dead at 39

The son of legendary singer Natalie Cole was found dead Monday night, and he was only 39 ... TMZ has learned.

Robert Yancy's body was found in his San Fernando Valley apartment when a friend who hadn't heard from him in a few days checked up on him.

Family members tell us ... authorities contacted them at 2:30 AM Tuesday and said Robert died of a heart attack. Law enforcement tells us, pending an autopsy, they have listed it as death from natural causes, but that can change once the autopsy and toxicology tests are completed.

The family tells us authorities were clear to them, drugs were not involved. As you know, Natalie famously fought drug addiction throughout her adult life until her 2015 death. Her cause of death was congestive heart failure.

Family members say Robert -- the grandson of Nat King Cole -- had never abused drugs and was "turning his life around." He worked in the music industry.

Robert's dad died of a heart attack at age 34, so it could be heart disease runs in the family.