Benzema, who ended a 14 years connection to Real this week, joins the Saudi Arabian champions on a three-year deal and will wear the number nine shirt.
"I am excited to experience a new football league in a different country. I have been fortunate to achieve amazing things in my career and achieve everything I can in Spain and Europe," Benzema said.
"It now feels the time is right for a new challenge and project ... I am looking forward to joining my new teammates and, together with them, help take this amazing club and the game in Saudi Arabia to new levels."
Having joined Madrid in 2009 from Olympique Lyonnais, Benzema became the leading attacker after Ronaldo left in 2018. He won the Champions League five times, the La Liga title four times and Copa del Rey three times during his time in Real Madrid.
He made 648 appearances and he is the second Real’s all-time goal scorer with 354. Only Ronaldo has more.
The 35-year-old won the Ballon d'Or award for the best men's player in the world, last year and he looked set to stay at the Spanish club for one more year, after a 2022-23 season in which he struggled with injuries and missed out on France's World Cup squad in Qatar.
Though, an offer estimated by the media to be worth more than 100 million euros from Saudi Arabia made him rethink his decision to stay in Spain for a final season.
Benzema held a goodbye ceremony at Real’s training ground on Tuesday.
“It hurts me because I had a dream in my head - to sign for Madrid, and finish at Madrid,” he said.
“But sometimes life gives you other opportunities,” he added.
“I am excited to experience a new football league in a different country,” Benzema said later.
“I have been fortunate to achieve amazing things in my career and achieve everything I can in Spain and Europe. It now feels the time is right for a new challenge and project,” the last year’s Ballon d’Or holder said later on.
Al Ittihad’s President, who is Anmar Bin Abdullah Alhailae, was present at the signing in Madrid and said “To sign the current Ballon d’Or holder from Real Madrid is a historic milestone for this special club,”
Al Ittihad named Benzema’s entrance “the most impactful transfer in the club’s history”, as it contests with Riyadh-based clubs Al Hilal and Al Nassr in their bid to sign football’s biggest names.