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.
well said, absolutely. I'm glad he hand picked from the sounds of it a much younger person he trusts to fill the job. That's what's up.