Struijk was born in Deurne, Belgium, and joined Leeds' under-23s from Ajax in January 2018 as an 18-year-old. He made his senior debut under Marcelo Bielsa in December 2019 and went on to spend eight and a half years at Elland Road, making 196 appearances and scoring 14 goals.
He won two Championship titles with the club and last season made 34 Premier League appearances as Leeds United finished comfortably in the top half in their first season back in the top flight after a three-year absence.
He captained the side on a number of occasions and served as vice-captain under Daniel Farke.
The sale is pure profit for Leeds, who signed Struijk from Ajax as a free agent and retained a sell-on clause from the deal.
Struijk's departure leaves Leeds short in central defence going into the new season, but the funds secured from the transfer help them remain compliant with Premier League profit and sustainability rules.
Brighton mamager Fabian Hurzeler was clear about what drew him to the player.
"Pascal is exactly the sort of profile we wanted to add to the squad this summer, in that he's got good Premier League experience and has his peak years ahead of him," Hurzeler said.
"He really showcased what he's capable of last season — great defensive actions while comfortable on the ball — with his style well suited to ours.
"He is also a leader, having worn the captain's armband at times for Leeds last season. He is an excellent addition to the group and I am looking forward to working with him."
Struijk becomes Brighton's second significant centre-back acquisition of the summer, following the club's £52m sale of Jan Paul van Hecke to Tottenham earlier in the window.