The 2002 World Cup winner's message, recorded at the request of Joao Jose Vasconcelos' family, urges the captors to free the engineer, according to brother Luis Henrique de Vasconcelos.
Ronaldo's gesture comes as the Arab Corporations Union in Brazil also lent their voice to the cause, appealing to the captors to release Vasconcelos.
The union said in a statement sent to Aljazeera that the Arab and Muslim communities in Brazil - numbering more than 12 million people - genuinely wished for the release of the Brazilian citizen.
The recording by Ronaldo, who plays for Spanish club Real Madrid, will be translated into Arabic and sent to television stations in the Middle East in a bid to help gain Vasconcelos' liberty, his brother said.
The family also recorded a message asking the captors to show clemency and release the 55-year-old engineer.
Vasconcelos' family on Sunday expressed the hope that the captors would show mercy, noting that Brazil had opposed the Iraq war.
Vasconcelos had been working for Brazilian construction company Norberto Odebrecht - one of Latin America's biggest construction companies - at a power station near Baiji when he was seized.
The captors said the ambush was carried out as a joint attack between the Mujahidin Brigades and the Army of Ansar al-Sunna, but included no threats or demands.
Earlier in the week, Brazilian Foreign Minister Celso Amorim said he would send Middle East special envoy Afonso Celso de Ouro Preto to Jordan to handle Vasconcelos' case.
Aljazeera had earlier aired a videotape by the group claiming to be holding the Brazilian. Although the tape did not show the engineer it included his identification card.
After the capture, Brazil advised all its citizens to stay out of Iraq.