The incident happened several hours before Portugal's decisive Euro 2016 Group F clash with Hungary in Lyon with a berth in the tournament's last 16 at stake.
The 21-second clip shows Ronaldo, surrounded by security guards, strolling by a lake with his team mates.
When the reporter from Portugal's CMTV approaches to ask if he was prepared for the game, Ronaldo grabs the microphone from him with one hand and pitches it into the water.
The 31-year-old Real Madrid forward has been under pressure after refusing to shake hands with Iceland's players following Portugal's 1-1 draw in their opening game and missing a penalty in their second group match -- a goalless draw with Austria.
Jose Delgado, a correspondent for Portuguese daily A Bola, told Reuters Ronaldo was frustrated by his poor performances.
"The hours before the game are always difficult for the players and I don't think it's a good idea they mix with journalists because things like this may happen," he said.
"Ronaldo has been anxious in the last few days because he didn't play well in the opening two games. Let's see what he can do against Hungary and if he can come to his usual level."
(Additional reporting by Philip O'Connor; Editing by Adrian Warner and Ken Ferris)
