The transfer is subject to the agreement of personal terms, visa and a medical, but the 36-year-old appears set to leave Juventus to re-join the club where he won eight major trophies between 2003-2009.
Initial media reports had said Ronaldo, who told Juventus coach Massimiliano Allegri he wanted to leave the Turin club on Thursday, was likely to move to United's rivals City, but Sky Sports said the Premier League champions had decided against doing a deal.
Fans in Manchester are ready for his return.
"Amazing mate. Honestly, I think if you go back 24 hours, a lot of United fans would have thought he's on his way to Man City, so, it was kind of, you know, that kind of, like thought of 'God, he can't go there'. But the news that's just broke today, unbelievable. Just blows everything out the water," Manchester United fan Kevin said on Friday.
Financial details were not disclosed, but the BBC claimed the fee was 15 million euros for Ronaldo up front, with eight million euros added on. Sky Italia said he will sign a two-year contract worth $660,048.00 (480,000 pounds) a week.