The actor was attending the third day of Comic Con Experience in Sao Paulo to promote his new film Free Guy.
As Reynolds jumped down from the stage to approach hundreds of fans, the barrier surged forward and almost fell on top of him before he managed to jump out of the way.
There was a commotion in the moments afterwards as staff rushed over to ensure Reynolds was unhurt.
Reynolds appeared unharmed as he composed himself and went to check on the wellbeing of his fans.
His Free Guy co-star Joe Keery (Stranger Things‘ Steve Harrington) had also jumped down to greet the crowd, but was seen being helped back onto the stage after the incident.
Free Guy is a sci-fi comedy about a bank cashier, who discovers he is a background character in a brutal, open world video game called Free City that will soon go offline.
Killing Eve star Jodie Comer and Channing Tatum also star in the film.
Free Guy will be released in UK cinemas on 1 July 2020.