Sancho was officially unveiled as a Red Devils player on Friday after completing a €85 million deal from Bundesliga giants Borussia Dortmund.
There were reports that Sancho was going take on the number seven jersey, but Edinson Cavani will keep the United's most famous jersey in his second season at Old Trafford.
As a result, the England international will now wear the number 25 this campaign, a number which was previously worn by Odion Ighalo, Antonio Valencia and former Bafana Bafana star Quinton Fortune.
"I'll always be grateful to Dortmund for giving me the opportunity to play first-team football, although I always knew that I would return to England one day," Sancho said.
"The chance to join Manchester United is a dream come true and I just cannot wait to perform in the Premier League. This is a young and exciting squad and I know, together, we can develop into something special to bring the success that the fans deserve. I am looking forward to working with the Manager and his coaching team to further develop my game."