The 29-year-old Sagan announced his decision during the presentation of the route of the 2020 Giro, at a televised event in Milan on Thursday.
He says: “I always said that I want to do the Giro before I finish my career.”
Next year’s Giro starts in Hungary, which borders Sagan’s native Slovakia.
Richard Carapaz, who became the first rider from Ecuador to win a Grand Tour with his triumph in this year’s Giro, said he wants to defend his title but that the decision ultimately lies with Team Ineos, which he will join from the start of next season.
He says: “I can’t guarantee it but I really think I will be there. I will do my utmost to be there.”