The hugely ambitious transfer is expected to heap unprecedented focus on Saudi soccer ahead of a potential bid to stage the World Cup in 2030, as well as on Al Nassr, which is little known outside of Asia.
For Ronaldo, however, it likely brings the curtain down on his career in elite club soccer.
Last summer he wanted to move to a club that was competing in the Champions League after United failed to qualify for Europe's biggest club competition.
Despite links with Chelsea, a transfer did not materialize.
He also endured a disappointing World Cup, which saw him dropped by Portugal for its last two games before leaving the field in tears following a loss to Morocco in the quarterfinals.
Ronaldo also only managed three goals for United in 16 games this season, with one of those coming from the penalty spot.However, there have still been landmarks.
He scored his 700th club career goal against Everton in October and became the first male player to score in five World Cups with his penalty in Portugal's 3-2 win against Ghana in the group stage.
Ronaldo will aim to win a league title in a fourth different country after winning seven domestic championships in England, Spain and Italy combined.
Al Nassr plays Al Ta'ee on Thursday, but it unclear if Ronaldo will make his debut so soon after completing his move.