The financial details were not disclosed but French media said the transfer fee was around 25 million euros (£21.47m).
"Al-Nassr has officially signed with the Ivorian star Seko Fofana. We wish him good luck with our stars," the club said on Twitter.
Fofana joined Lens in 2020, where he scored 21 goals and provided 11 assists in 112 appearances, helping the club consolidate their place in the French top flight.
"Seko arrived when the club was freshly promoted, with this ambition to help it regain its influence and the continental scene," Arnaud Pouille, Lens managing director said in a statement.
"Everyone at the club says thank you and shows their respect for what has been achieved. And, proof of his deep attachment to Racing, he will very soon become the first player leaving a club to become a shareholder," he added.
The midfielder will join Cristiano Ronaldo, Marcelo Brozovic, fellow Ivory Coast international Ghislain Konan, and David Ospina, as the Saudi Pro League club has become an attractive destination for Europe-based players.
Al Nassr, who finished second in the league last season, were banned from registering new players due to outstanding debts, FIFA said last week, until they paid a sum of 460,000 euros to Leicester City for the signing of Nigerian attacker Ahmed Musa.
FIFA added that the relevant bans will be lifted immediately upon the settlement of the debts being confirmed by the creditors concerned.
