But where does the 24-year-old Dominican Republic-born defender feature in the list of United's costliest signings?
We run through the top ten most expensive players to join Leeds in terms of transfer fees in reverse order - and seven of that top ten are part of Marcelo Bielsa' s current first team squad.
Upon joining the club on a four-year deal, Firpo told LUTV: "I'm really happy, I wanted to be here," he said.
"I always wanted to play in the Premier League, which for me is the best league in the world.
"For me to be here in this club, a historic club with really, really good fans. I'm excited to play here.
"This summer for me many clubs were interested in me but when Victor (Orta) spoke with me and showed me the project, I said wow it's interesting, they really want me to play, if I play well."
1. 10 - Jack Harrison
The winger finally joined Leeds on a permanent deal after two years on loan at the club from Manchester City for a fee of around £11m last week. Photo by LUFC.
2. 9 - Robbie Fowler
The clinical striker joined Leeds from Liverpool for £11m back in November 2001, a fee that would be much higher today in real terms given inflation. Photo by Gary M. Prior/ALLSPORT via Getty Images.
3. 8 - Robbie Keane
Republic of Ireland international forward Robbie Keane signed for Leeds in a deal worth around £12m in May 2001 having initially joined on loan from Inter Milan. Photo by Laurence Griffiths /Allsport via Getty Images.
4. 7 - Robin Koch
The German international defender joined Leeds for a fee in the region of €13m in August 2020. Picture by LUFC.
5. 6 - Junior Firpo
The left back joined Leeds from Barcelona for €15m on Tuesday and goes comfortably into the top ten of most expensive signings made by the Whites. Picture by LUFC.
6. 5 - Helder Costa
Leeds signed winger Costa on a permanent deal from Wolves last summer in a deal worth around £16m. Costa had spent the previous season on loan at Elland Road. Photo by OLI SCARFF/POOL/AFP via Getty Images.