Delays in stadium construction, infrastructure issues and protests have marred the lead-up to the FIFA World Cup, which starts on June 12.
Ronaldo, who is a member of the organising committee, said the nation’s people had been let down.
“I’m ashamed for the Brazilian people who were expecting big investments and a great legacy for them, for us,” he said.
“We promised to rebuild airports and improve transport and infrastructure. I am sorry for all the things we promised and have not completed.
“We have some numbers – only 30 percent of all the things that were promised will be done in time for the World Cup to get underway. That number is my only concern, for that I’m ashamed.
“The Brazilian people are the ones affected by our failure.”
Ronaldo, the World Cup’s all-time leading goalscorer and two-time winner, has big hopes for the national team.
The 37-year-old believes Brazil, who face Croatia, Mexico and Cameroon in Group A, come in with more confidence than the victorious 2002 team he was a part of.