Experts believe that he will remain the most influential person in the country.
Raul Castro is preparing to step down as head of the Communist Party of Cuba, the most prominent position in the country.
While the move is historical, few expect significant changes for ordinary Cubans.
Raul Castro and his late brother Fidel have been in power since the 1959 revolution.
In 2018, Castro Jr. said he would retire in 2021 and expected to be succeeded by Diaz-Canel.
Diaz-Canel, 60, represents a new generation and was elected president in 2019 for the first of two possible five-year terms.
Many analysts are convinced that Castro, who turns 90 in June, will remain the most powerful figure on the island until his death.