Financial details were not disclosed but British media reported Al Nassr paid a transfer fee of 27.5 million euros for the defender who had two years left on his City contract.
Laporte is Al Nassr's sixth major foreign signing in the close season after Sadio Mane, Marcelo Brozovic, Alex Telles, Seko Fofana and Otavio. The club signed Cristiano Ronaldo last season.
"I am proud to have represented Manchester City over the last six seasons... I will always be a City fan and I look forward to seeing you all again," he said.
Signed in the January transfer window of the 2017/18 season from Athletic Bilbao, Laporte went on to become an integral member of City's back line and made 180 appearances for the Premier League side in all competitions.
He won five Premier League titles, two FA Cups, three League Cups and the Champions League during his time at City. But he fell down the pecking order last season with Ruben Dias, Manuel Akanji and Nathan Ake preferred in central defence.
Laporte posted a farewell message on social media on Wednesday, thanking the club's fans for their support.
Despite playing for France's youth teams, he switched allegiance to Spain in 2021 and has 22 caps for the national team.
City also lost Riyad Mahrez to the Saudi Pro League after Al Ahli snapped up the Algerian winger.
But City were given a boost when Bernardo Silva, who joined City in 2017 and was linked with a move to Paris St-Germain, signed a contract extension until 2026.
