The Cuba Debate outlet said the 68-year-old had been hospitalized and then continued receiving treatment as an outpatient. The family will make the funeral arrangements, it said.
"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," Cuba Debate outlet said.
Fidel Angel Castro Diaz-Balart, also reportedly known as "Fidelito" because he looked much like his father, was a nuclear physicist who studied in the Soviet Union.
Recent years, he had been working as a scientific counselor to the Cuban Council of State and Vice-president of the Cuban Academy of Sciences.
From 1980 to 1992, Fidel Angel Castro Diaz-Balart was in charge of the national nuclear programme in Cuba and forged the development of a nuclear power plant in the country. However, Havana had to halt this project in 1992 due to the lack of funding.
His father, Fidel Castro, died in 2016 at the age of 90.