TEHRAN, Young Journalists Club (YJC) - The nuclear scientist, also known as "Fidelito", or Little Fidel, because of how much he looked like his father, had initially been hospitalized and then continued treatment as an outpatient.
"Castro Diaz-Balart, who had been attended by a group of doctors for several months due to a state of profound depression, committed suicide this morning," Cubadebate website said.
Fidelito, who had the highest public profile of all Castro's children, was born in 1949 out of his brief marriage to Mirta Diaz-Balart before he went on to topple a U.S.-backed dictator and build a communist-run state on the doorstep of the United States during the Cold War.
Through his mother, Castro Diaz-Balart was the cousin of some of Castro's most bitter enemies in the Cuban American exile community, U.S. Representative Mario Diaz-Balart and former U.S. congressman Lincoln Diaz-Balart.
He was also the subject of a dramatic custody dispute between the two families as a child.
Cuba scholars say his mother took him with her to the United States when he was aged five after announcing she wanted a divorce from Castro, while he was imprisoned for an attack on the Moncada military barracks in Santiago.
Castro was able to bring Fidelito back to Cuba after the 1959 revolution.
A multilingual nuclear physicist who studied in the former Soviet Union, Castro Diaz-Balart was head of Cuba's national nuclear program from 1980 to 1992, and spearheaded the development of a nuclear plant on the Caribbean's largest island until his father fired him.
Fidelito had been working for his uncle President Raul Castro as a scientific counselor to the Cuban Council of State and Vice-president of the Cuban Academy of Sciences at the time of his death.