The move ends Struijk's seven-year spell at Elland Road, where he made 196 appearances after joining from Ajax in 2018.
The 26-year-old featured in 34 Premier League matches last season as Leeds secured a 14th-place finish on their return to the top flight.
Brighton turned to Struijk after selling fellow centre-back Jan Paul van Hecke to Tottenham for 52 million pounds recently.
"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," head coach Fabian 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."
Leeds confirmed the deal includes a sell-on clause and said they had offered Struijk an improved contract before respecting his desire for a new challenge.
The club added that the transfer would ensure they remain compliant with Profit and Sustainability Rules after keeping their key players last summer to boost their survival hopes.
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