"One world where we all love our children. Please help. #Rohingya #Refugee," he wrote, adding a link to a page by nonprofit Save the Children where people can make online donations to the cause.
The picture shows a father and child at a health clinic run by the charity in a refugee camp in Bangladesh.
The Rohingya Crisis Children’s Relief Fund will help provide refugee children and families with water, food and shelter.
Additionally, he posted a photo of himself with his four children.
Ronaldo is working with the child rights charity. Previously, he has donated food, clothes and medical aid to families affected by the conflict in Syria.
Since Aug. 25, over 656,000 Rohingya have crossed from Myanmar's western state of Rakhine into Bangladesh, according to the UN.
The refugees are fleeing a military operation in which security forces and Buddhist mobs have killed men, women and children, looted homes and torched Rohingya villages.