Finalising one of the summer's most high-profile deals on the last day of the European transfer window, the five-time world player of the year completed his sensational return to United after 12 years away and said: "I'm back where I belong".

"History has been written in the past," the 36-year-old Ronaldo wrote on Instagram on Tuesday, "and history will be written once again! You have my word!"

Ronaldo signed off his post by dedicating his return to Old Trafford to the man who first brought him to United in 2003 - when he was a skinny, supremely gifted 18-year-old winger - and turned him into a superstar.

"Sir Alex," he wrote, "this one is for you."

A 15 million euros ($A28.5m) agreement was sealed with Juventus over Ronaldo's transfer on Friday, and one of soccer's most stunning deals in years was completed after a medical.

"I can't even start to explain my feelings right now, as I see my return to Old Trafford announced worldwide... ," Ronaldo wrote.

"This is absolutely 100% the stuff that dreams are made of!"

With a striker not a priority until Ronaldo became available, United found itself with a squad overloaded with attacking players.

That prompted the club to sell winger Daniel James to Leeds United Leeds for a reported fee of 25 million pounds ($55.3m).

A number of other teams were looking to reshape their squads on transfer deadline day, too.

Chelsea further bolstered their strong squad by signing Spain midfielder Saul Niguez on loan from Atletico Madrid until the end of the season.

Tottenham and Serge Aurier parted ways while the club completed the transfer of Emerson Royal from Barcelona, who only signed the Brazilian right-back back from Real Betis at the start of June.

West Ham signed Croatia attacking midfielder Nikola Vlasic on a five-year deal from CSKA Moscow - two days after they signed centre-back Kurt Zouma from Chelsea.

Leicester signed winger Ademola Lookman on loan from German club Leipzig and sent Belgium midfielder Dennis Praet on loan to Italian team Torino.

Crystal Palace signed French striker Odsonne Edouard from Celtic while Everton picked up veteran Venezuelan striker Salomon Rondon from Chinese club Dalian on a free transfer.

Arsenal announced it signed Japan defender Takehiro Tomiyasu from Italian club Bologna while Hector Bellerin and Reiss Nelson were sent out on loan to Real Betis and Feyenoord respectively.

Australian Associated Press