The Five-time Ballon d'Or winner says he's back where he "belongs".
Manchester United have paid an initial 15m euros ($17.7M) to sign 36-year-old Cristiano Ronaldo from Juventus, a decade after he left the club to join Real Madrid.
He has signed a two-year contract, with the option to extend for another year.
On Instagram, Ronaldo said: "Everyone who knows me, knows about my never ending love for Manchester United," and hinted that Sir Alex Ferguson had a big role in the transfer.
Ronaldo had been linked with Manchester City but opted to return to United, where he spent six years and won three Premier League titles, one Champions League, two League Cups and one FA Cup - as well as the Fifa Club World Cup and Community Shield.
Ronaldo could be available for United's home game against Newcastle United on 11 September, which is the club's first match following the international break.
It's unclear if he'll return to wearing the iconic number 7 shirt - currently held by Edinson Cavani.