Media reports have claimed the 37-year-old Portugal international could earn up to $200 million a year from the move, but he will miss out on the chance to extend his record as the all-time leading scorer in the Champions League with his record currently standing at 140 goals.
Messi is on 129 goals in the competition.
Ronaldo is also unlikely to add to his Ballon d’Or collection - the trophy awarded to the best player in the world.
Meanwhile, Messi will be among the favourites to win that trophy for an eighth time next year after his World Cup triumph.
He also has the chance to win the Champions League with Paris Saint-Germain, having lifted European club soccer’s biggest prize on four occasions with Barcelona.
Ronaldo won the Champions League five times during spells with United and Real Madrid.
Six months ago Ronaldo wanted to join a team playing in the Champions League after United failed to qualify for this season’s competition.
However, a move never materialized, with the most serious interest in him coming from an unnamed Saudi Arabian club.
It is not known what other serious offers were made after he became a free agent last month, but the move to Al Nassr represents a significant step down compared to the level he has operated at throughout his career.
Still it is a major coup for soccer in the Middle East and will add to the debate over Saudi Arabia’s attempts to use so-called “sportswashing” to improve its reputation internationally after its sovereign wealth fund led a buyout of Premier League club Newcastle United last year.
“This is more than history in the making. This is a signing that will not only inspire our club to achieve even greater success but inspire our league, our nation and future generations, boys and girls to be the best version of themselves,” Al Nassr posted on social media.
Ronaldo remains one of soccer’s biggest global icons, but the move comes at a time when he has faced questions over his ability to still produce his best form at the highest level.
He managed just three goals for United in 16 games this season, with one of those coming from the penalty spot.
He became the first male player to score in five World Cups with his penalty in Portugal’s 3-2 win against Ghana in Qatar, but ended the tournament having been dropped for his country’s last two games before going out to Morocco in the quarterfinals.
By comparison Messi inspired Argentina’s third World Cup triumph, scoring seven goals, including two in the 4-2 penalty shootout win against France in the final after a 3-3 draw through extra time.
Ronaldo and Messi have had their achievements compared throughout their careers.
Messi’s World Cup win saw him emulate soccer greats Pele and Diego Maradona by lifting the sport’s biggest prize.
To many, it will also have given him the edge in his personal rivalry with Ronaldo. And at the age of 35, he still has time to further embellish his career in top level soccer.
Meanwhile, Ronaldo has earned headlines in recent months for his antics away from the field.
He was dropped and made to train away from United’s first team when refusing to come on as a substitute in a game against Tottenham in October.
He then conducted an explosive interview with Piers Morgan in which he criticized manager Erik ten Hag and United’s owners the Glazer family.
It led to the termination of his contract, his eventual move to Al Nassr and the next chapter of his career away from the glare of top flight European soccer.
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