The club confirmed Wales international James had signed on Tuesday afternoon with the fee being around £28 million.
United welcomed their bitter rivals to Old Trafford on August 14 in their Premier League opener, with Leeds fans aiming a chant at James.
:rotating_light::soccer: ' NEW: A video of Leeds United fans singing 'You're too shit to play for Leeds' to Dan James pic.twitter.com/1gqSDFxI3I
- Football For All (@FootballlForAll) August 30, 2021
Marcelo Bielsa's side lost that game 5-1 and a section of their support decided to sing: "You're too s***, you're too s***, you're too s*** to play for Leeds."
Leeds have been linked with James before and he even held up their shirt in leaked images.
He was set to join from Swansea City in 2019 before the Welsh outfit decided against selling the winger.
"Whilst from a fan's perspective it clearly looked like a f*** up, I think we were just the unwitting victim of boardroom politics at the Swansea end," club chief Angus Kinnear said at the time.
"It was unfortunate because we couldn't make the deal work for Daniel in the end and I think it would have been a good deal for him and his family, but also from a football perspective."
James instead joined United in summer 2019 and failed to nail down a regular starting spot.
He started life in the Premier League brightly, scoring in each of his first three games.
The arrivals of Jadon Sancho and Cristiano Ronaldo this summer however means game time would be limited.
James' last game for United came in Sunday's 1-0 win over Wolverhampton Wanderers.
Victor Orta, Director of Football at Leeds, said: "'I have to say a huge thank you to my team firstly - the football department have been monitoring Daniel for a number of years and every member of my team has worked hard to get this deal over the line.
"It is no secret that Daniel is a player we have been keen to acquire for a number of years and today we achieved our goal.
"As a player we feel Daniel is an excellent fit for a Marcelo Bielsa team, he is quick, direct and works hard - we look forward to him joining up with the squad, finally!"
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