The agreement extends the Englishman’s stay in the Italian capital after initially arriving on loan in August 2019 from Manchester United.
“Choosing to remain at Roma was an easy decision to make,” Smalling told Roma’s website.
“I’m playing the best football of my career here and my family and I have felt so welcome in this wonderful city from day one.”
Since joining, Smalling has established himself as not only one of Roma’s most key defenders, but all of Serie A.
“During his time at the club, Chris has distinguished himself not only for his performances on the field but also for the sense of belonging he has shown, which has led him to tie his future to the club,” said general manager Tiago Pinto.
“We are delighted to be able to count on both his defensive prowess and his leadership skills. He is a key dressing-room figure and a role model for the younger lads in our squad.”