Kossounou joins Atalanta on a season-long loan.
La Dea are paying €5.5m for the loan and can turn the deal into a permanent transfer for €25m. The outright option isn't obligatory.
Kossounou is now the club's eleventh summer arrival.
The defender joins Rui Patricio, Juan Cuadrado, Raoul Bellanova, Lazar Samardzic, Marco Brescianini, Mateo Retegui, Nicolo Zaniolo, Ibrahim Sulemana, Ben Godfrey and Charles De Ketelaere as new signings.