The Catalan side have opted to pay his release clause, which stands at €39.5million, and he will move north immediately.
The Frenchman has signed a five-year contract, with a buyout clause set at €300million.
“Clément Lenglet is one of the most promising defenders on the planet,” Barcelona said in a statement.
“With ten appearances for the French national team and a height of 186 cm (6 ft 1 in), Lenglet arrives at Barça after success in Seville, where led the defense with elegance and determination.
“Lenglet, who is left-footed, stands out for his good touch and his combination of strength, intelligence, and, above all, ball skills.”