Arnaiz passed a Barca medical yesterday, signing a three-year deal with the option for another two seasons.
The contract carries a €20m buyout clause, which will double to €40m by year three.
Barca have paid Valladolid an initial €3.4m for the striker, with the fee rising to €4m based on appearances and success.
Arnaiz will intially begin his Barca career with Gerard Lopez’s B team.