Ronaldo will return to Old Trafford on an initial £13 million (€15million/$18million) deal, with add-ons of up to £7million (€8million/$9million).
The 36-year-old took to social media to thank Juventus fans for their support after he spent three seasons with the Bianconeri.
“Today I depart from an amazing club, the biggest in Italy and surely one of the biggest in all of Europe,” he said on Instagram.
“I gave my heart and soul for Juventus and I’ll always love the city of Turin until my final days.
“The ‘tifosibianconeri’ always respected me and I tried to thank that respect by fighting for them in every game, every season, every competition. In the end, we can all look back and realise that we achieved great things, not all that we wanted, but still, we wrote a pretty beautiful story together.