The 21-year-old forward spent the first half of the season at Chelsea, before returning to his parent club and joining the Whites until the end of the 2025-26 campaign.
Buonanotte came to England at the age of 18 from Argentinian club Rosario Central and went on to make 50 appearances for the Seagulls.
He has since had loan spells with Leicester City, where he featured 35 times across all competitions, scoring six goals, as well as with Chelsea, with whom he played eight times, scoring once, providing two assists. He last appeared in the 5-1 FA Cup third round win over Charlton on Saturday.
Chelsea and Brighton both agreed that the switch would be beneficial to the player's ongoing development.
Brighton sporting director, Jason Ayto, said, "Facundo wants to play and he demonstrated in his loan to Leicester City last season the impact he is able to have in the Premier League when he does. This is a great move for all parties and we looking forward to seeing how he continues his development with Leeds United. We wish him well for the remainder of the season and look forward to welcoming him back in the summer."
