Nianzou leaves as a free agent under a contract rescission agreement rather than for a fee, with Sevilla still on the hook for a share of the defender's costs this year through amortisation and part of his substantial salary.
The Andalusian club will cover half of his remaining wages but still expect to gain around €1m of salary cap space as a result of the departure, having been operating over their limit.
The Frenchman's spell in Seville is widely regarded as one of the worst value-for-money signings in the club's history. He arrived in the summer of 2022 for €16m from Bayern Munich but lost roughly two-thirds of his available games to persistent muscle injuries.
By some estimates, each appearance he made for Sevilla ended up costing the club in the region of €1m, while his rising annual salary became an increasingly heavy burden on their wage limit throughout his four years at the club.
Nianzou's time in Andalusia was also short on standout performances during his limited opportunities. Among the low points was a red card in last season's match against Oviedo at the Carlos Tartiere, which came during Luis Garcia Plaza's first game in charge of the club. Despite the struggles, Sevilla thanked Nianzou in their official statement for his willingness to help facilitate the exit, consistently describing the move as a transfer throughout their communication.
Now 24, Nianzou will hope a return to his native France can revive a career that once looked destined for the top. A product of the Paris Saint-Germain academy, he moved to Bayern Munich as a free agent in 2020 before his switch to Sevilla two years later.
Lille, who finished third in Ligue 1 last season and secured Champions League qualification, are said to see the centre-back as a squad reinforcement rather than a project for the future.