"Fidel Castro Diaz-Balart, who had been treated by a group of doctors for several months due to deep depression, took his life this morning," Cuba's official newspaper Granma reported.
"Fidelito" -- as he was known on Caribbean island nation -- had initially been hospitalized for his condition and then continued with outpatient follow-ups.
Born on September 1, 1949, he was the son of the revolutionary Castro's first marriage to Mirta Diaz-Balart.
The younger Castro was very popular in Cuba even though he never held a political position. He bore a strong resemblance to his father: both were tall and had bushy beards.
The younger Castro was always faithful to his father's communist ideals.
His death came 15 months after that of his father at the age of 90.
At that funeral, Castro was seen standing alongside five half-siblings that his father had with another woman, Dalia Soto del Valle.
A scientist who studied physics in the former Soviet Union, Fidel Jr helped usher in the development of a nuclear power program in the Communist-ruled country.
In his most recent public appearances Castro gave lectures on nanoscience, in which he specialized.
Funeral arrangements will be made by the family, state media said.
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