Ronaldo, 33, returned to form ahead of the blockbuster Champions League showdown against Paris Saint-Germain with a hat-trick as struggling Liga holders Madrid trounced Real Sociedad 5-2 on Saturday.
Asked about Ronaldo and the reports of his unhappiness at Santiago Bernabeu, Marcelo told Esporte Interativo : "I don't think there is any truth to that, as if he was unhappy then he wouldn't be here.
"He does everything to help the team. He's a mature guy, who has a very strong squad around him at Real Madrid.
"I don't think he's unhappy. The world knows what he is capable of doing, just look at his history with the club.
"I don't think he can be dissatisfied."
Ronaldo has scored 23 goals in 28 games in all competitions this season, including 11 in 18 in La Liga.