Leite joined Union Berlin from Porto in the summer of 2022, initially on loan before the move was made permanent, and went on to make 136 appearances for the German club, scoring twice and adding four assists. He chose not to renew his contract, which expired at the end of June, leaving him a free agent over the summer before Lazio moved to secure his signature.
The Portuguese international, whose full name is Diogo Filipe Monteiro Pinto Leite, has signed a contract running to the summer of 2028 with an option for a further year, according to Lazio's official announcement.
He arrives in Rome to link up again with Danilho Doekhi, his former Union Berlin centre-back partner, who also joined Lazio on a free transfer this summer after the pair spent four seasons together marshalling the Bundesliga club's defence.
Lazio have made an ideal start to the season, winning all three of their opening Serie A fixtures, a run matched only by Roma and Inter among the division's other early pace-setters.
Leite becomes one of several new arrivals in the Italian capital this summer, joining a group that also includes Davide Frattesi, Andrea Pinamonti, Josip Sutalo, Alfonso Pedraza and Bruno Galassi alongside Doekhi, as the club looks to build squad depth across multiple positions following a busy transfer window.
Before his time in Germany, Leite progressed through Porto's academy and spent a loan spell at Braga, establishing himself as a composed, left-footed defender comfortable building attacks from the back, qualities that persuaded Lazio to move for him once his availability as a free agent became clear.
His partnership with Doekhi is already well established after 108 competitive matches together at Union Berlin, including 94 in the Bundesliga and further appearances in the Europa League and DFB Pokal, giving Lazio a defensive pairing with genuine understanding built up over several seasons rather than one they will need to develop from scratch.
Signing him on a free transfer also means Lazio avoid negotiating a fee with his former club, adding further value to a deal that gives head coach Gennaro Gattuso a proven, experienced option to call upon immediately at the back.