The Red Devils announced this across their social media channels on Friday.
The 36-year-old is back at Old Trafford 12 years after leaving the club to join Real Madrid.
Ronaldo will sign a two-year contract.
United posted a collage of Ronaldo from his previous playing days at the club along with the words: “Welcome home, @Cristiano.”
The Portugal captain scored 118 goals in 292 games in his first stint at United, winning the Premier League title three times and the Champions League in 2008.
He left for Real Madrid in 2009 for a then world-record fee of £80 million but will now make an emotional return.