Koulibaly has been a regular fixture in Carlo Ancelotti's team this season, making 34 league appearances so far.
The 27-year-old has managed 125 clearances, 31 blocks in the Italian top-flight this season and he has also won 67 percent of his total tackles .
His defensive contributions helped Napoli manage 13 clean sheets in the league as they sit second behind leaders Juventus, with two matches remaining.
Koulibaly will be presented with his award at Stadio San Paolo on Sunday when Napoli host Inter Milan for their penultimate league fixture of the season.
🥇— Koulibaly Kalidou (@kkoulibaly26) May 18, 2019
Best defender in @SerieA 2018/19: a title that makes me proud. A great satisfaction to share with my teammates, coach, club and supporters: thanks to everyone 💪🏿
🇸🇳 #KK26 💙 #famiglia @sscnapoli 🇮🇹 #seriea 🙏🏿 pic.twitter.com/qHunlD83z5