The move is reported to have cost the Nou Camp outfit €35m in a five-year deal after spending just 18 months in Spain following his arrival from Nancy in 2017.
Lenglet's deal includes a release clause, which stands at €300m (£264m/$350m), and runs until June 30, 2023.
Lenglet, 23, departs Sevilla after two seasons having become a key player at the Ramon Sanchez Pizjuan Stadium-he made 35 La Liga appearances last term.
The Rojiblancos were ultimately forced to back down as the centre-back failed to find an agreement over a new contract, though they were adamant that Barca would pay his release clause.
The youngster has also been sorted after by Premier League giants Manchester United but he preferred to join Barca.
Lenglet, a former France under-21 worldwide, becomes Barca's second signing of the summer following confirmation last week that Gremio midfielder Arthur Melo will also join the club this month.
Mina's the favourite to be allowed to go - be that on loan or permanently with a buy-back option - but Paulinho's return to China means Barca no longer need to free up a non-EU spot in the squad.