At least 15,000 children are among the more than 300,000 people who have been killed in the Syrian war.
"Hello, this is for children of Syria. We know that you have been suffering a lot. I am a very famous player, but you are the true heroes," the Real Madrid and Portugal star said in a video message for Save The Children posted on his social media.
"Don't lose your hope, the world is with you. We care about you, I am with you," added the footballer.
Nick Finney, of Save the Children's Syria campaign, said "Ronaldo is not only one of the world's most iconic sports figures, but he also has become a beacon of hope for millions of boys and girls worldwide - and hope is something that Syria's children need now more than ever."
At least 15,000 children are among the more than 300,000 people who have been killed in the Syrian war over the past five years, according to aid groups.