The World Cup-winning goalkeeper spent most of his career at the Santiago Bernabeu and is a legend with Los Blancos, but left the club under a cloud in 2015 when he joined Porto and was not given a proper send-off.
Asked in an interview with DjMaRiio whether he would like to go back, he said: "One day... why not? I consider it to my home, even though I have been away for five years.
"The connection is palpable. I was there for 25 years. Of course I'm convinced that one day I'll return to the club."
Casillas also called for former team-mate Zinedine Zidane to be given time in his second spell at Real.
"We have become very demanding with this Real Madrid," he said. "We have to give it time and that is down to Zidane and the people at the club."