Valero, 27, would shortly travel to Italy for a medical and to tie up the details of the move, Villarreal, who were relegated from La Liga last season, said on their website.
A club spokesman declined to confirm media reports that the deal was worth seven million euros.
An attacking midfielder with an eye for goal, Valero came through Real Madrid's youth academy before joining Real Mallorca and later had a stint at English club West Bromwich Albion. He has made one appearance for Spain.
Fiorentina, which is owned by the Della Valle family of Tod's shoemaking fame, finished 13th in Serie A last term.
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