The 27-year-old signed a four-year contract that runs until 30 June 2022.
“I cannot wait to get started and show everyone what I can do,” Santon said.
“Coming to Roma means a lot to me, I really wanted to make the move and being here now has given me a lot of motivation.”
Santon: "I cannot wait to get started and show everyone what I can do.
— AS Roma English (@ASRomaEN) June 26, 2018
Santon has been capped eight times by Italy – making his debut for the Azzurri as an 18-year-old in 2009.
“We know Davide and his ability very well, but what struck me most is his desire for a resurgence in his career,” Roma sporting director Monchi said.
“Davide is a versatile player, and the coach will be able to count on a lad who can comfortably play on both flanks.”