The Brazilian stopper is set to join the Italian capital club just days after impressing for Timao in the Copa Libertadores final against Boca Juniors
The 25-year-old will sign a four-year contract at the Stadio Olimpico after the two clubs agreed on a reported €5 million deal.
"Everything is settled with Roma," Castan told Radio Bandeirantes. "It is a great offer. I leave Corinthians with the feeling that I did a good job.
"I spoke a lot with my family before making a decision. I am 25 years old and I am at a good moment in my career."
Castan, who will now embark on his second second spell in Europe after previously representing Swedish side Helsingborgs, joined Corinthians from Barueri in 2010.
However, he only cemented his place in the Timao first team in 2011 following Willian's retirement, and went on to make 110 appearances for the club that won the Copa Libertadores on Wednesday.