Manchester United have reportedly completed a £64m deal, to sign Koulibaly from Napoli.
Koulibaly has also been linked with Tottenham and Arsenal, but it is claimed that United have moved quickly to land the defender.
The 28-year-old is however not expected to join until the summer
Italian journalist Fabio Santini broke the news, and speaking on the TV show Il Processo he said:
"Napoli will sell Kalidou Koulibaly to Manchester United for a figure of around 70-75m euros. It seems all done.
"Naturally, such a deal will be closed only in June and certainly not in January in the winter transfer window."
The signing would be the second time within a year United have spent big money on a centre-back, after Harry Maguire joined from Leicester City for £80m last summer.