The Argentine follows Fer Niño and Javi Morcillo through the door this summer, with sporting director Christian Bragarnik continuing to reshape a squad that still needs further additions before the transfer window closes.
Buonanotte, 21, joins Martín Anselmi's attacking midfield options after Brighton agreed to loan him out for the season, giving the club a young, talented player with plenty of room to develop ahead of Elche's 26th campaign in the top flight, which begins away at Deportivo La Coruña on 17 August.
Brighton signed the playmaker, who also holds an Italian passport, from Rosario Central for around €6m in early 2023, a deal that covered 80% of his rights, after he had made 32 appearances and scored four goals for the Argentine club.
He has since made 50 official appearances for the Seagulls, five of them in the Europa League, showing enough to be considered one of Argentine football's brightest prospects, and remains under contract at the Amex until 2028.
Loan spells have followed in each of the past two seasons in search of regular football: he made 35 appearances and scored six goals for Leicester City in 2024/25, before starting last season at Chelsea, where he made his Champions League debut, only for a lack of opportunities to send him to Leeds United in January, where he managed just four appearances.
Buonanotte has represented Argentina at various youth levels and has already won two senior caps for his country.
In its welcome statement, Elche described him as a versatile player capable of operating as an attacking midfielder, winger or inside forward, highlighting his technical quality, vision, one-on-one unpredictability and knack for creating chances as qualities that should add creativity and dynamism to the attack.