The 20-year-old, who spent last season on loan at Leicester City, joins after skipping his flight to West Yorkshire, where Leeds United were reportedly ready to snap him up.
The Argentine attacker has not featured for The Seagulls in recent weeks amid speculation surrounding his future.
Speaking to the club website, he revealed that he cannot wait to get started and help the side in any way he can on and off the pitch.
“I’m very happy to join Chelsea," said Buonanotte.
"It’s a great opportunity for me and I’m just looking to add as much to the squad as possible, help the team and help the staff."
Buonanotte’s move represents the latest in a long line of deals between Chelsea and Brighton, which also includes Moises Caicedo, Robert Sanchez, Marc Cucurella and even manager Graham Potter back in 2022.
The 20-year-old managed six goals and three assists in 35 appearances last season as the Foxes were relegated from the Premier League.
He will help provide cover for the recently injured Liam Delap and the upcoming departure of Nicolas Jackson to Bayern Munich.