James, 21, joined the Red Devils for an expected £15m fee from Swansea City.
On Friday, he posted a confirmation of the transfer on his Instagram story.
His girlfriend Ria Hughes shared the picture on her own story, adding three gold star-eye emojis.
Ria, a sports instructor from Wales, has been one of her boyfriend’s biggest supporters, long before his senior debut for Swansea in August 2018.
In March, after James scored his first senior goal for Wales in a 1-0 Euro 2020 qualifier win over Slovakia, the stunning blonde took to Instagram to praise him.
She wrote: “Well done today and always, first goal for Wales, you work so hard everyday and deserve everything! So proud of you!”
As well as showing her support for James, she often shares incredible selfies with her 3k followers.
One admirer wrote under a snap: “You lil fitty.”
Last year, the pair bought their first home together near Swansea, but are now planning for a move to Manchester.
“You work so hard everyday and deserve everything! So proud of you!”
Ria Hughes, girlfriend, Daniel James
The highly-rated winger has been linked with a move to United for some time, after impressing this season in the Championship, scoring four goals and providing seven assists.
Ole Gunnar Solskjaer will welcome the first transfer of the summer, as he looks to overhaul his squad with fresh young talent after a disappointing sixth-place finish in this season’s Premier League.
Man Utd fans have also been quick to welcome Ria to Old Trafford along with her boyfriend.
Under a recent Instagram holiday snap, one fan wrote: “Welcome to Manchester United, biggest club in the world.”
Last month, the move was delayed in tragic circumstances after James’s dad died suddenly ahead of his transfer.
Earlier this week, James shared an emotional tribute to his dad on Instagram.
Posting four pictures of father and son, James wrote: “There is not one day goes by that I don’t miss you, you are my inspiration and I wouldn’t be where I am today without you.”
He went on: “Words can’t describe the pain I’m feeling now, I just want you to know how much I love you and how much you will be missed by all of us.”