Harries only made his professional debut in the final game of last season, and has gone on to make nine senior appearances this campaign.
Caretaker boss Mark Venus has moved to safeguard one of Coventry's brightest young stars, telling the official website: "The club are trying to build a solid foundation for the future, and doing that by investing in both homegrown and acquired talent on long-term deals.
"We are grateful that Cian and his advisors have put faith in the football club and have committed to a future here.
"Cian is another one of our talented academy graduates to sign an extended deal and that is a show of belief in the club model.
"I would like to thank everybody involved in the negotiation process and look forward to watching Cian develop his career this season and beyond."