Koulibaly is not the first Chelsea player to make the move to Saudi Arabia. N’Golo Kante agreed to join Al-Ittihad on a free transfer following the expiration of his deal with the Blues.
A number of high-profile names have moved to the Saudi Professional League during the past six months.
Karim Benzema has already agreed to join Al-Ittihad after 14 seasons at Real Madrid, following Cristiano Ronaldo who moved to Al Nassr from Manchester United in December.
Wolves captain Ruben Neves has also agreed a £47m move to Al-Hilal.
After Koulibaly and Kante, Chelsea outcasts Edouard Mendy, Romelu Lukaku and Pierre-Emerick Aubameyang have also been linked with moves to the Gulf state.