The Portuguese insisted he had multiple offers from elsewhere following his messy exit from Manchester United - but he had given his word to Al Nassr and wanted a 'new challenge'.
Among the options, he said, was Australia, though it is hard to envisage there was a bid that would have tempted him financially. Ronaldo's contract in Saudi Arabia is reported to be worth more than $A300m-a-year, for two-and-a-half years.
The deal is expected to include promotional work for Saudi Arabia, who are considering a bid to host the 2030 World Cup, and Ronaldo added: "I want to give a different vision of this country and football. This is why I took this opportunity."
Prior to joining Al Nassr Ronaldo had said he wanted to join a team that played in the Champions League. That could yet happen as Spanish sports daily Marca reported on Tuesday there is a clause in the contract allowing him to be loaned to Premier League side Newcastle United if the Magpies qualify for next season's competition.
Both Al Nassr and Newcastle are majority-owned by the Saudi sovereign wealth fund.
Whether Newcastle manager Eddie Howe would welcome having to integrate Ronaldo, who is 38 next month, into his team is unknown.
Ronaldo, who has won five Ballon d'Or awards for the best player in the world and five Champions League titles, said at his unveiling on Tuesday: "I'm so proud to make this big decision in my life. In Europe my work is done. I won everything and played for the most important clubs in Europe. This is a new challenge.
"Nobody knows this, but I had many opportunities in Europe, Brazil, Australia, America, Portugal, many clubs tried to sign me. I gave my word to this club.
"I'm grateful to Al Nassr for giving me this opportunity to develop football for the younger generation, for the women as well. I feel very happy and proud."
He batted away questions about criticism of his move, saying, "I'm a unique player, for me it's normal."
Ronaldo flew in to Riyadh's King Khalid International Airport on Monday night, where he and his family were greeted by fans, before undergoing medical tests on Tuesday.
He was then presented to fans at the club's 25,000 capacity Mrsool Park in Riyadh. They loudly chanted his name as he walked around signing footballs and firing them into the stands, which were a sea of yellow and blue.
"Me and my family, we want to say thank you to all of you for this support and your welcome," an emotional Ronaldo said after being handed a microphone.
Al Nassr play Al Ta'ee on Thursday, but it unclear if Ronaldo will make his debut so soon.
His last match, for Portugal in the World Cup, ended with him leaving the field in tears following a loss to Morocco in the quarter-finals.
His last club match, for Manchester United on 6 November, ended with a 3-1 defeat to Aston Villa. He had been restored to the side, and made captain, after being dropped for leaving Old Trafford early after refusing to come on as a substitute against Tottenham.
Sixteen days later his contract was ended by mutual agreement with immediate effect after he gave an interview criticising the club's manager Erik ten Hag and owners, the Glazer family.
- with agencies
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