Ronaldo was without a club after his contract with Manchester United was terminated in November following a blockbuster interview with Piers Morgan.
Ronaldo criticised senior figures at the club, including manager Erik Ten Hag, who he declared he had no respect for, after losing his place in the side this season.
Al Nassr are nine-time champions of Saudi Arabia and are currently second on the league standings.
Ronaldo is a five-time Ballon D'or winner and has also won league titles in England, Spain and Italy.
The 37-year-old has won the UEFA Champions League five times and captained Portugal to European championship glory in 2016.
"History in the making," Al Nassr tweeted.
"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. Welcome Cristiano to your new home Al Nassr FC."
Al Nassr Chairman Musalli Al-Muammar added: "This deal is more than just writing a new historical chapter. This player is a higher example for all athletes and youth in the world, and with his presence in Al-Nassr, we will move forward to achieve more success for the club, Saudi sports, and future generations."
"I am eager to experience a new football league in a different country. Al Nassr Club's vision is very inspiring," the Saudi outfit quoted Ronaldo as saying.
"I am fortunate that I have won everything I set out to win in European football and feel now that this is the right moment to share my experience in Asia."
