At 17 years old, she was a finalist in the 2004 Summer Olympics in Athens, where she finished 5th in 400-metre individual medley, the best placement obtained by a Brazilian woman swimmer at all times. Also ranked 7th in 4×200-metre freestyle, and 11th in the 200-metre individual medley. Joanna Maranhão broke the South American record of the 4×200-metre freestyle with a time of 8:05.29, along with Paula Baracho, Mariana Brochado and Monique Ferreira, record that was not broken until today; she also broke the South American record of the 200-metre individual medley, with a time of 2:15.43, at semifinals. At the 400-metre individual medley, the Argentine swimmer Georgina Bardach broke the South American record and won the bronze medal doing 4:37.51; Maranhão broke the Brazilian record with 4:44.00, record that was not broken until today.