The 21-year-old, capable of playing both left-back and centre-back, was a key player for Rayo Vallecano last season, popping up with two goals in his 28 games.
Hull have now confirmed Mendy’s signing, joining the club on a five-year deal until the summer of 2031, with a club option of a further year.
Although the fee remains undisclosed, Hull have confirmed it’s a club record deal, breaking it for the second time they’ve done so this summer.
It’s been reported the fee is £22 million.
Speaking to the club’s website, Mendy said: "I am very happy to be here and very proud to be here as well.
"I am a very athletic player, and I am a defender at heart. I also really enjoy the game and coming out from the back cleanly and aggressively when needed.
"I have wanted to play in the Premier League since I was little, so it’s a great challenge to face and I am ready for it.
"This season is going to be a good season, and we will all be in it together."