The 29-year-old touched down in Italy in January to spend the second half of the 2022-23 season on loan with the Giallorossi, helping Jose Mourinho's side reach the final of the Europa League.
Llorente made 12 appearances for Roma across all competitions during his six-month spell, starting five Serie A games and playing 15 minutes in their Europa League final defeat to Sevilla.
The Spain international largely struggled to break up the established order of Gianluca Mancini, Roger Ibanez and Chris Smalling, but he was still regarded as an important member of Mourinho's setup.
Back in December, Llorente penned a contract extension with Leeds until the summer of 2026, but he has swiftly returned to Italy following the Whites' relegation from the Premier League.
"I'm proud to be able to remain part of the Giallorossi family. When I had the chance to come to Rome in January, I didn't hesitate for even a second and even less so during this period of waiting," Llorente told asroma.com.
"I've always known I'd do everything possible to stay, not just to remain part of such a historic club, but also to be back alongside my team-mates to fight together, with our fans getting behind us, to achieve the targets that we deserve. I'm sure we can accomplish something fantastic together."
Meanwhile, managing director Tiago Pinto added: "Despite arriving halfway through the season, Diego needed very little time to get fully settled into the dressing room and meet the expectations that we had for him on the pitch.
"That's why we're thrilled to be able to keep calling upon his attributes and his wealth of international experience."
Llorente previously came through the youth academy at Real Madrid and represented Rayo Vallecano, Malaga and Real Sociedad before Leeds paid £17.1m for his services in 2020.
The Spaniard has since scored four goals in 59 appearances for the Elland Road outfit in all competitions, but he started just seven Premier League matches in the first half of the 2022-23 season.
Llorente briefly broke back into the Spain national team setup owing to his form at Leeds, but the 10-cap international has not represented his country since June of last year and was not called up to the Roja squad for the 2022 World Cup or recent Nations League finals.
Leeds have also allowed fellow centre-back Robin Koch to join Eintracht Frankfurt on loan for the 2023-24 campaign, while Tyler Roberts, Joel Robles and Adam Forshaw have all left the club permanently.
Meanwhile, Llorente becomes Roma's third signing of the summer window following the arrivals of Evan Ndicka and Houssem Aouar on free transfers.