The Portugal superstar, 37, has signed for Al-Nassr until 2025, announcing at his unveiling that his "work is done" in Europe.
Amnesty International says Ronaldo's signing is part of a "wider pattern of sportswashing" in Saudi Arabia, a country which has backed a breakaway golf league, hosted high-profile boxing bouts and Formula One grands prix.
It is also interested in co-hosting the men's football World Cup in 2030.
"Instead of offering uncritical praise of Saudi Arabia, Ronaldo should use his considerable public platform to draw attention to human rights issues in the country," Dana Ahmed, Amnesty's Middle East researcher, said.
"Saudi Arabia regularly executes people for crimes including murder, rape and drug smuggling. On a single day last year, 81 people were put to death, many of whom were tried in grossly unfair trials.
"The authorities are also continuing their crackdown on freedom of expression and association, with heavy prison sentences handed down to human rights defenders, women's rights activists and other political activists.
"Cristiano Ronaldo should not allow his fame and celebrity status to become a tool of Saudi's sportswashing. He should use his time at Al Nassr to speak out about the myriad of human rights issues in the country."
Ronaldo's salary at Al Nassr has been reported by a number of outlets as being upwards of £160million a year.
At his first public appearance for his new club, Ronaldo told a press conference on Tuesday: "I'm feeling very good. I'm so proud to make this big decision in my life.
"In Europe, my work is done; I won everything, I played at the most important clubs in Europe and for me now, it's a new challenge.
"This is a great opportunity. Nobody knows this but I had many opportunities in Europe, many clubs in Brazil, in Australia, in the US and even in Portugal.
"Many clubs tried to sign me but I gave my word to this club to develop not only the football but other parts of this amazing country."
Ronaldo wore a suit as he addressed the assembled media, with the only questions coming from an Al Nassr official, before changing into his new club's kit for his presentation at Mrsool Park stadium.
Ronaldo added: "I'm a unique player. I beat all the records there. I want to beat a few records here. This contract is unique but I'm a unique player, so for me it's normal.
"Many people speak and give their opinions but they really do know nothing about football. It's not the end of my career to come to Saudi Arabia. I really don't worry about what the people say."
Ronaldo, whose United contract was terminated in November shortly after criticising the club in a TV interview with Piers Morgan, was greeted by thousands of ecstatic fans as he walked on to a stage on the Mrsool Park pitch.
He said in a brief on-stage interview "I will give my best for this club" before posing for photographs and then autographing and kicking balls into the assembled crowd.
Chants of 'Ronaldo' and his trademark 'siu' celebration echoed constantly around the stadium before the presentation ended with Ronaldo joined on the pitch by his children and partner Georgina Rodriguez.
Al Nassr head coach Rudi Garcia, formerly of Roma, Marseille and Lyon, hailed Ronaldo as a terrific coup for the club.
"Cristiano is one of the best players in the world, he's a legend," Garcia said. "In my life, I've seen that the great, great players like Cristiano are the easiest to manage.
"We are here to win, nothing else."