Brazil international defender Castan was a key member of a Roma team who finished second in Serie A last season - conceding just 25 goals in the process - and started all but two of their top-flight fixtures.
And the 27-year-old, whose previous contract still had two years left to run, has been rewarded with a fresh deal that will keep him in the Italian capital until June 2018.
Roma president James Pallotta told the club's official website of his delight at securing the long-term future of one of coach Rudi Garcia's outstanding performers.
"We are extremely pleased and honoured to have extended his contract for another two years," he said.
"He is one of the main reasons why our defence was so strong last season."
Castan, who has two senior caps for Brazil, joined Roma from Corinthians in 2012.
The news of his renewal comes as a boost to the club, with fellow defender Mehdi Benatia having been left less than impressed with his own offer of a new deal as rumours of summer exit continue to build.