Igor made 42 appearances across all competitions last season as Fiorentina reached the finals of the Europa Conference League and Coppa Italia, although they lost both to West Ham United and Inter Milan respectively.
The 25-year-old initially joined Fiorentina from SPAL in January 2020.
He has also played for Liefering, Salzburg, Wolfsberger and Austria Vienna.
Igor has signed a four-year contract at the AMEX Stadium and boosts a Brighton defence that will compete in the Europa League next season.
"I'm very happy and excited to start playing as soon as possible. I'm happy to be here," Igor told Brighton's official website.
"I want to be here to play for the coach that wanted me and the style of play at Brighton also fits well with my own style.
"It's a childhood dream to play in the Premier League, I'm very excited. It's always been my goal to play here. I'm really happy to reach this level.
"As always, I'm going to play the hardest as possible on the pitch. I'll do my best every day, but most importantly to try and win the heart of the fans.
"It's always good to have an experience in European competition. It's my first time playing in the Europa League but I'm hoping to add to the team as much as possible and play as hard as I can to help in our European journey.
"I can't wait to train and play for Roberto. He's a fantastic coach and I think I can evolve so much under him. I want to become one of the best defenders in the team. When he left Italy he already had a big name, but after what he has achieved here his acknowledgment in Italy is even bigger."
