Sport says Barca have been following him for several years and have been keeping a close eye on his progression in Ligue 1.
Diop's a tall centre-back (1.90m), who moves quickly with and without the ball. The 21-year-old is rated at around 25 million euros and there's plenty of interest in him.
He is one of the young French defenders Barça have been monitoring.
Barça are also alert to the German market. Schalke's Thilo Kehler, 21, has been followed as has Leipzig's Dayot Upamecano.