Mr Castro, 89, told a party congress that he is handing over the leadership to a younger generation "full of passion and anti-imperialist spirit".
His successor will be voted in at the end of the four-day congress.
Although Mr Castro has not endorsed a successor, it is widely believed the party leadership will pass to Miguel Díaz-Canel, who took over as the island's president in 2018.
dude fought in the revolution, can't really get a better leader than the one who helped shape cuba's socialism. so badass