Lens captain Fofana heads for Saudi club Al-Nassr
"I leave you with the feeling of accomplishment and pride at being a part of this history which is ours," Fofana, 28, wrote in a farewell video posted on Instagram.
The Ivorian international midfielder played 112 times for the northern French outfit scoring 21 goals in three years.
He departs as Lens return to the Champions League for the first time in 21 years, following an exceptional season during which they finished second in Ligue 1 one point behind Paris Saint-Germain.
"Seko is here," Al-Nassr wrote on Twitter.
Fofana joins Ronaldo, Croatian international Marcelo Brozovic and Colombian goalkeeper David Ospina at the Riyadh-based club.
Other big names who have headed for Saudi Arabia in recent weeks include French players Karim Benzema and N'Golo Kante at Al-Ittihad with Senegalese Kalidou Koulibaly and Portuguese Ruben Neves playing at Al-Hilal.
Fofana, who captained Lens 72 times, was one of the best players in Ligue 1 last season.
He was the only midfielder shortlisted for the best player in the championship alongside four forwards -- Kylian Mbappe, Lois Openda, Jonathan David and Lionel Messi.
"I want, on behalf of the club, to especially salute an exemplary captain who has been sparkling and has shown leadership for three seasons," said RC Lens general manager Arnaud Pouille.
"Proof of his deep attachment to Racing, he will very soon become the first player leaving a club to become a shareholder," he said.
Fofana spent his youth career at Paris FC, his hometown club, and Lorient, before signing his first contract with Manchester City in 2014, but never played a professional match for the club.
Loaned to Fulham then Bastia, he then played four years for Serie A club Udinese before joining Lens, just promoted to Ligue 1, in the summer of 2020.
RC Lens also announced the departure of Polish international Adam Buksa, on loan to Turkish club Antalyaspor with the option to buy.