Lenglet, who only joined Sevilla from French side Nancy in January 2017 after just half a season of top flight football under his belt, has agreed a five-year deal until 2023 and will have a €300m buyout clause in his new Barcelona contract.
The 23-year-old was a standout performer for Sevilla last season, missing just three La Liga games and playing 54 times in all competitions. He was particularly key when Sevilla knocked Manchester United out of the Champions League at the last 16 stage.
Lenglet is set to become the second new face to arrive at Barcelona this summer after Brazilian midfielder Arthur completed a €30m move from Gremio earlier this month.
Andres Iniesta, Gerard Deulofeu and Paulinho are the only departures so far. More transfers, both incoming and outgoing, are expected in the coming weeks.