At least 10,000 people used the chatrooms, with some paying up to $1,200 (£1,000) for access.
Some 74 people, including 16 underage girls, were exploited.
“The accused has widely distributed sexually abusive content that he created by luring and threatening many victims,” the Seoul Central District Court said on Thursday, according to Yonhap news agency.
It said Cho was found guilty of violating laws to protect children from sexual abuse and for running a criminal ring that produced and sold abusive videos in order to make a profit.
Cho’s criminal syndicate sold the videos it acquired through blackmail to secretive chatrooms on the Telegram app.
The case sparked a national outcry in South Korea.
In March, a police committee took the unusual step of naming Cho, a 25-year-old college graduate, after five million people signed petitions asking for his anonymity to be lifted.
“I apologize to those who were hurt by me,” Cho had said that month as he was led away from a Seoul police station. “Thank you for putting a brake on the life of a devil that could not be stopped.”
Police have said at least 124 suspects were arrested and 18 operators of chatrooms on Telegram and other social media, including Cho, were detained following investigations into similar sexual crimes since late last year, reported Reuters news agency.
Five other defendants have received sentences ranging from seven to 15 years.