The decision follows a controversial interview in which the 37-year-old criticised the club and said he had “no respect” for manager Erik ten Hag.
Both parties stated that Ronaldo’s exit was “mutually agreed”.
“The club thanks him for his immense contribution across two spells at Old Trafford,” said a Manchester United statement.
They wished “him and his family well for the future” and added “everyone at Manchester United remains focused on continuing the team’s progress under Erik ten Hag and working together to deliver success on the pitch”.
Ronaldo is with Portugal at the World Cup in Qatar and is set to captain them in their opening Group H game against Ghana on Thursday.
“Following conversations with Manchester United we have mutually agreed to end our contract early,” said a statement from the forward.
“I love Manchester United and I love the fans, that will never ever change. However, it feels like the right time for me to seek a new challenge.
“I wish the team every success for the remainder of the season and for the future.”