The 20-year-old Argentine was left out of the Blues’ European squad, a decision that came shortly after he had spoken openly about his dream.
Buonanotte arrived at Stamford Bridge with a strong reputation, having made 50 appearances for Brighton with five goals and already earned two senior caps for Argentina.
“It’s a massive step in my career,” he told club media.
“I’m really happy to be here and ready to offer as much as I can to the team and the staff. I’m sure it will be a positive season.
“Of course, I’ll get the opportunity to play in the Champions League for the first time, which will be a great challenge. I’m really looking forward to this season and hopefully I can help the club to achieve its goals."
While Buonanotte misses out, Chelsea have still secured spots for other high-profile additions. Alejandro Garnacho, Romeo Lavia, Tyrique George, and Josh Acheampong have been included.