The 29-year-old Argentine has just helped his side capture the Copa Libertadores, and his performances have not gone unnoticed, with the Rome giants moving swiftly to secure his signature.
"Forza Lazio! I'm happy to be here," Gentiletti told Lalaziosiamonoi.it.
"This is a great opportunity for my future."
Lazio president Cladio Lotito was similarly pleased with the deal, and was full of praise for Gentiletti's abilities.
"Gentiletti is strong and quick," Lotito said.
"It was our coach Stefano Pioli who chose him."