The 29-year-old has made a total of 59 appearances for the West Yorkshire outfit across all competitions since moving to Elland Road from Real Sociedad in the summer of 2020.
However, Llorente has failed to nail down a regular place in Jesse Marsch's starting lineup in the 2022-23 campaign, featuring in only two of Leeds' last 13 Premier League games.
The Spain international has fallen behind Liam Cooper, Robin Koch and January signing Maximilian Wober in the centre-back pecking order at Elland Road and was keen to seek for regular first-team football elsewhere.
Llorente now reunites with Roma head coach Jose Mourinho, who handed the central defender his professional debut during his time at Spanish giants Real Madrid in 2013.
Speaking to Roma's official website on his arrival at the club, Llorente said: "When the possibility of coming to Roma was presented to me I did not need to think about it twice. Coming here is a big step forward for me in my career and I want to make the most of this opportunity.
"Rome is beautiful, an incredible city, and I think it is almost impossible to find a better place to play football. The club is fighting to achieve great things on all fronts this season and I am ready to make my contribution towards that."
Roma's general manager Tiago Pinto added: "Diego Llorente offers characteristics that will complement those of his new teammates well and for that reason we are all convinced that the club's defensive ranks will benefit from his arrival.
"He is a player who learned his trade at one of the finest academies in football and in the years since then has gained a range of experience that will allow him to confront this new challenge in the best manner possible."
Llorente becomes Roma's second signing of he January window after winger Ola Solbakken, who joined the Italian giants from Bodo/Glimt earlier this month.
Meanwhile, Leeds have completed the signing of centre-back Diogo Monteiro from Swiss outfit Servette, and the 18-year-old will link up with the club's Under-21s squad.