Speculation over Ronaldo's future has grown in recent months, with the 32-year-old understood to still be upset by allegations of tax evasion from the Spanish authorities last summer.
Madrid's form on the field is also thought to have left the five-time Ballon d'Or winner disillusioned in the Spanish capital, but Marcelo insists that Ronaldo is happy at the club.
"I don't think there is any truth to that, as if he was unhappy then he wouldn't be here," he told Esporte Interativo.
"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."