United will pay Sporting Lisbon an initial £47m for the 25-year-old Portugal international, although with add-ons the fee could jump to £67.6m.
A deal between the two clubs was agreed on Wednesday, with personal terms and a medical completed today.
“I’m delighted that we’ve agreed [the deal] with Sporting,” said United manager Ole Gunnar Solskjaer.
“There have been good negotiations with the club and we know, through history, these two clubs have been good with each other.”
Fernandes was officially announced as a United player on Thursday when it was announced that he would wear the number 18 shirt at Old Trafford.
At Sampdoria he wore the 10 shirt, however that number is occupied by Marcus Rashford at United.
At Lisbon he wore the number 8 – currently occupied by the Spaniard Juan Mata – while for the Portugal national team he has worn the number 16, taken by Marcos Rojo.
The number 18 shirt was most recently worn by former club captain Ashley Young, who left to join Italian side Inter Milan earlier this month.
Before Young it was worn by Frazier Campbell – now of Huddersfield Town – and, most famously, Paul Scholes.
The Manchester United legend wore the shirt in the 1996/97 season, briefly switching to number 12 for the next campaign, before reverting to 18.
He wore the shirt up until his retirement in 2011. For his brief return, he took the number 22.