Ronaldo de Assis Moreira was born in the Rio Grande do Sul capital of Porto Alegre. His mother, Miguelina, is a former salesperson who studied to become a nurse. His father, João, was a shipyard worker and footballer for local club Esporte Clube Cruzeiro (not to be confused with Cruzeiro EC), and suffered a fatal heart attack when Ronaldo was eight. After Ronaldo's older brother, Roberto, signed with Grêmio, the family moved to a more affluent home in Porto Alegre, which was a gift from Grêmio to convince Roberto to stay at the club. Roberto's career was ultimately cut short by injury.

Ronaldo's football skills began to blossom at an early age, and he was first given the nickname Ronaldinho because he was often the youngest and the smallest player in youth club matches. He developed an interest in futsal and beach football, which later expanded to organized football, and his first brush with the media came at the age of thirteen, when he scored all 23 goals in a 23-0 victo