The Brazilian, born in Uba on 2 January 2005, made 27 appearances for Gremio last season, scoring twice.
Fiorentina paid a total of €17 million for the deal, including €15 million plus €2 million in add-ons, making Viery the most expensive central defender in the club's history.
He becomes the latest addition to a back line being reshaped by new coach Fabio Grosso. Shortly after the move was confirmed, Viery posted a video on his Instagram account welcoming himself to his new club.
The clip opens with panoramic shots of Florence, before moving on to the Stadio Artemio Franchi and the atmosphere generated by the Curva Ferrovia.
It closes with footage of Viery himself out on the pitch, accompanied by the message "Forza Viola".
Alongside the post, the defender shared his first words as a Fiorentina player.
"I'm really happy about this new phase of my life and my career! It's an honour to wear this shirt. Forza Viola," Viery wrote.
The reel gives Fiorentina supporters an early glimpse of their new signing before he has kicked a ball for the club, with the Brazilian already embracing the culture around the Franchi.
His arrival completes a significant outlay in defence for Fiorentina this summer, with the club making clear through the size of the fee how highly they rate the 21-year-old's potential.
Grosso will now have the opportunity to work with Viery through pre-season as he looks to integrate his new centre-back into a reshaped Fiorentina defence ahead of the 2026-27 campaign.
The fee underlines the ambition behind the move, with Fiorentina willing to break their transfer record for a defender to bring in a player they see as a long-term building block at the back.
Viery's move to Tuscany follows a breakthrough season in Brazil, where his performances for Gremio were enough to convince the Serie A club to move for him this summer.
His social media message, filmed against the backdrop of the Franchi and the club's most passionate supporters, suggested a player keen to build an immediate connection with his new fanbase before pre-season has even begun.
For Fiorentina, the deal represents both a statement of intent in the transfer market and a clear signal of the direction Grosso wants to take his defence as he begins shaping the squad for the season ahead.