In a social media post the club said, "The world's greatest athlete officially signed for Al Nassr".
Speaking in the club shop where fans were already buying shirts with Ronaldo's name and number on, Al Nassr fan Ashraf Allhammad said he hopes the signing will bring more recognition for the club.
"Hopefully this deal will be a success for Al Nassr and for the club's future vision and marketing for the club with an international fame," he said.
"Having Ronaldo will boost the value of the Saudi league, if you see the tweets and views for this matter, all foreigners talking about this deal," said Khaled Almajed.
Ronaldo, 37, left Premier League giants Manchester United last month following an explosive television interview in which the forward said he felt betrayed by the club and did not respect their Dutch manager Erik ten Hag.
He represented Portugal in Qatar, where he became the first player to score in five World Cups after netting a penalty in his side's opening Group H game against Ghana. Portugal were knocked out in the quarter-finals by Morocco.
Ronaldo said Qatar would probably be his last World Cup as he plans to retire at 40, with the move to Saudi Arabia likely to mark the swansong in the career one of the game's greatest current players alongside Lionel Messi.