There have been reports linking Ronaldo with a move away from Turin this summer but Agnelli said the Portugal international will remain at the club.
"I am entirely convinced he will stay," he said. "I think the report was down to an interview from several months ago, which just happened to be published just before we played against a French team.
"It's an old media trick. No, I can assure you that Ronaldo remains a pillar of Juventus."
The club announced on Saturday that manager Maurizio Sarri had been sacked despite winning a ninth successive Serie A title last month.
Sarri was under pressure throughout the campaign after failing to convince the critics during his first season in charge.
Information from Reuters was included in this report.