The 18-year-old will officially move to Stamford Bridge next off-season.
Ahanor, an Italian international who primarily operates at left-back, has emerged as a regular starter for Atalanta after joining the club from Genoa last off-season. The transfer reportedly cost 16 million euros initially, with a further 4 million in potential add-ons.
The youngster made 28 Serie A appearances last season and also gained Champions League experience, making his debut in Atalanta's 2-1 win over Club Brugge last September. His progress earned him a first senior Italy call-up in May for friendly fixtures against Luxembourg and Greece.
Chelsea's interest in Ahanor initially developed around the time they signed Atalanta right-back Marco Palestra in July, and the Blues have now secured a long-term commitment from the defender.
The move represents another step in Chelsea's squad-building programme under Xabi Alonso, who took charge at Stamford Bridge in July.
The manager has overseen several additions, including Morgan Rogers from Aston Villa for a club-record 117 million pounds, while experienced Premier League figures Danny Welbeck and Jordan Henderson have also been brought in.
The Blues have also sanctioned a departure, with striker Marc Guiu joining RB Leipzig in a 14.6-million-pound deal including add-ons. The 20-year-old arrived from boyhood club Barcelona in July 2024 and went on to make 29 first-team appearances for Chelsea, scoring eight goals.
