The 24-year-old has put pen to paper on a four-year deal at Elland Road, keeping him with the club until 2021.
However, he will spend the first of those years on loan at Spanish side Cultural Leonesa, having been shipped out on loan for the 2017-18 campaign.
Buoy, who can play as a centre-back or as a defensive midfielder, joined Juventus in 2012 but made just one senior league appearance for the club.
The Dutchman spent the majority of his time out on loan following his arrival from the Ajax youth academy, enjoying spells at the likes of Brescia, Hamburger SV, Panathinaikos, PEC Zwolle and Palermo.
Buoy becomes Leeds's ninth signing of the summer under new manager Thomas Christiansen.