Ronaldo and club president Florentino Perez appeared in the Royal Box at the Santiago Bernabeu on Sunday afternoon to announce the new five-year deal, with Real releasing a picture of the signing on their official Twitter account.
Marca reported that the 28-year-old's contract means he will eclipse Barcelona star Lionel Messi as the highest earner in Spanish football, with a new salary of £14.25 million a year for the next five seasons. Messi is reported to earn a fixed £10.9 million plus an extra £2.5 million in incentives. .
Ronaldo, who had two years remaining on his previous deal, had been linked with a move away from Madrid in the summer, with the likes of United and Paris Saint-Germain reported to have shown interest.
But speaking at the announcement of his new contract, he said: "I am staying here for the fans, and it could be for the rest of my career.
"I will be honest with you - everyone knows I was in Manchester for six years. Manchester is in the past.
"Now my club is Real Madrid. This is my home, my family is here and I'm really happy here. I'll be here five more years. I just want to win trophies for this club.
"I respect all the clubs who ask about me. But they always know that my decision, that my only goal is to be here and to play at this club until, maybe, the end of my career.
"The future nobody knows."
He said the agreement had been reached "at the end of the season - I then gave my word to Real, and the chairman his word to me".
Ronaldo scored for Madrid in their 2-2 draw with Villarreal on Saturday, with debutant Gareth Bale having netted the first.