The England international, 30, joins the club on a permanent basis from Premier League side Manchester United, for a fee of €15 million.
Smalling has agreed a three-year contract with the club that runs until 30 June 2023.
“Nothing in life worth having comes easy,” Smalling said.
“My head and my heart has always been in Rome. I am delighted to now be yours. Daje Roma!”
Smalling spent the 2019-20 season on loan with the club, making 37 appearances in all competition – and scoring three goals.
“We are pleased to be able to welcome Chris back to the club, as one of the pillars of our squad,” said Guido Fienga, the club’s CEO.
“I would like to thank Manchester United for the consideration they have shown throughout this strenuous and lengthy negotiation.”
Smalling will once again wear the No. 6 shirt this season.
Welcome back, Chris!
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