Selles has left Reading to pen a deal with the Tigers to 2027.
"I am really happy. When the possibility to come here became real, my eyes were on Hull City," the Spaniard told the club's website.
"I know the team, the club, the history and I'm looking forward to starting my journey with the team.
"I saw a lot of similarities with where we are as technical staff and what we want to do for the present and the future, and that's why I decided to come.
"From now, we go all in, all together, every single time, 100%. There is no tomorrow; every day is important, every action is important. We're going to fight for every inch together."
Reading have named Noel Hunt as Selles' successor.
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