The terms of the deal remain undisclosed.
Estupinan, 27, made 104 appearances for Brighton across all competitions, scoring five goals. Since joining the club from Villarreal in July 2022, he quickly established himself as a key figure at left-back, contributing both defensively and in attack during a transformative period for the Seagulls.
He was an integral part of the Brighton side that reached European competition for the first time in club history during the 2023/24 campaign. Last season alone, Estupinan featured in 34 Premier League matches and remained a consistent performer under both Roberto De Zerbi and current head coach Fabian Hurzeler.
Speaking on the move, Hurzeler praised the Ecuadorian's impact and professionalism:
"I have enjoyed working with Pervis and he has made a significant contribution to the club during his time here," Hurzeler said. "He is at the stage of his career when he wants a fresh challenge, and this opportunity to play in Serie A gives him that. I'd like to thank Pervis for his contribution and wish him all the best for the future."
Estupinan made his Brighton debut in a 2-0 win over West Ham United in August 2022 and quickly won plaudits for his pace, crossing ability, and tactical awareness. His move to Milan offers a new challenge in one of Europe's most competitive defensive leagues, as the Rossoneri look to strengthen their squad ahead of the new Serie A season.
The transfer marks another significant outgoing for Brighton, who have continued to evolve their squad under a model focused on talent development and timely sales.