The Senegal defender became the latest big name to ditch European football for the Middle East over the weekend.
Koulibaly has joined former Wolverhampton Wanderers captain Ruben Neves at Al-Hilal, signing a three-year deal.
His exit from Stamford Bridge comes just a year after he was signed from Napoli for £30million.
However, Koulibaly struggled to adapt to English football, making just 23 league appearances and he was also sent off in just his third match.
Following his move, the 32-year-old fired a parting shot at Chelsea in an interview with Corriere dello Sport.
“I had no guarantees to get regular playing time [at Chelsea],” he explained. “I was always professional but I don’t like to sit on the bench doing nothing.
“I prefer a place where I am wanted and at the centre of the project and can be an example for young players. The Premier League is fantastic and there are great players.
“They [Chelsea] expected the Koulibaly of Napoli, but I think my season was not so bad. I needed time and in one year I couldn’t prove what I wanted because of the coach’s and club’s choices.
“I am happy with what I learned about my limits, my family and my kids. My time had come and I had to make a decision.
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“My teammates sent me so many messages, they are all sorry, but I have to prepare for the AFCON. It’s the most important thing for me, Senegal want to win again, but it won’t be easy.”
Koulibaly fell out of favour under caretaker boss Frank Lampard last term and was not in Mauricio Pochettino’s plans for this season.
Pochettino still has Trevoh Chalobah, Benoit Badiashile, Wesley Fofana and Thiago Silva at his disposal in the centre of defence.
Chelsea will also welcome back loanees Levi Colwill, Ethan Ampadu and Malang Sarr this summer.
Koulibaly’s Saudi move has seen him follow former team-mate N’Golo Kante to the Middle East.
Hakim Ziyech and Edouard Mendy are also set to swap West London for Saudi Arabia in the coming days.