Lenglet is Barca's second signing of the summer after they captured midfielder Arthur Melo.
Barca believed that would pave the way for negotiations with Sevilla - instead of paying the buyout clause, they were hopeful of striking a deal which would have seen them pay more but with the fee spread across a number of instalments.
It has been claimed that Barca have activated the Frenchman's €35 million (R553 million) release clause after Sevilla refused to negotiate with the Blaugrana.
The La Liga champions triggered Lenglet's €35.9m release clause before agreeing terms with the centre-back on a five-year contract, which includes a new release clause of €300m.
The 23-year-old made the switch to Spain in January 2017, leaving Nancy, for whom he was captain - and seamlessly fitting into Jorge Sampaoli's team.
Manager Ernesto Valverde will likely use France youth global Lenglet alongside Thomas Vermaelen as back-ups for starting centre-back duo Gerard Pique and Samuel Umtiti.
The youngster has also been sorted after by Premier League giants Manchester United but he preferred to join Barca.
- Giant black granite sarcophagus unearthed in Egypt
- Israel to launch moon mission from Florida in December
- Chelsea confirm Antonio Conte's sacking with terse statement
- NASA may have discovered, accidentally destroyed organics on Mars in 1976
- Malankara Orthodox Church priest held on rape charge
- Jubilant Croatia fans revel in World Cup semi-final win over England
- Brooklyn Nets trade Jeremy Lin to Atlanta Hawks
- Richard Bacon In Medically Induced Coma
- Game of Thrones racks up 22 Emmy nominations
- Chris Wallace Announces Exclusive Interview With Vladimir Putin Monday After Trump Summit