The 23-year-old Norwich academy product joins the Elland Road outfit on a season-long deal with a view to a permanent move, becoming the fifth signing of the summer for the Whites.
Adeyemi failed to break into the Canaries’ first-team over his three-year spell, leading to loan stays at Bradford, Oldham and Brentford – the latter of which he was managed by current Leeds boss Uwe Rosler.
A season as vice-captain at Birmingham followed, before Cardiff brought him to south Wales for £1million last summer.
The midfielder struggled to make his mark during his time at the Cardiff City Stadium, leading him to joining what he calls “the perfect club”, on a temporary deal.
Speaking to Leeds United’s official website, the midfielder said: “I first heard it was a possibility last week and over the weekend it became clear that it could happen.
“With Leeds being a massive club and the manager being here and having worked with him before, it was the perfect club to come to.
“Cardiff said they were happy for it to happen and at that point it was an easy decision for me to get on the motorway.
“I’m really happy to be here. I’m glad it’s all gone smoothly and I’m just looking forward to getting out there and getting the season started.
Of his reunion with previous boss Rosler, Adeyemi said: “He had me on loan at Brentford from when I was at Norwich when I was younger, maybe three years ago now.
“I enjoyed working with him, he’s a really good manager. The opportunity to work with him again was one which I was happy to take.
“We had a little chat about the way he sees the team lining up and where I would possibly fit into that.
“He says there’s a number of different options of ways he wants to play.
“He made it clear that – like every other player – I’ll have to fight for my place and make sure I do well to keep it.”
Rosler himself waxed lyrical over the capture of Adeyemi: “He will add physicality, dynamism, good athleticism, height and footballing skills to our midfield.
“We have different types of midfielders now and it makes me excited to work with all of them.”