Monteiro qualified for two swimming events at the 2004 Summer Olympics in Athens, by clearing a FINA B-cut of 1:50.93 (200 m freestyle) from the World Championships in Barcelona, Spain. In the 200 m freestyle, Monteiro challenged seven other swimmers on the fifth heat, including three-time Olympian Jacob Carstensen of Denmark. He touched out Poland's Łukasz Drzewiński to take a fifth spot and twenty-ninth overall by 0.12 of a second in 1:51.78. He also teamed up with Adriano Niz, João Araújo, and Miguel Pires in the 4 × 200 m freestyle relay. Swimming the lead-off leg in heat one, Monteiro set his own personal best of 1:50.43, but the Portuguese team settled only for seventh place and fourteenth overall in a new national record of 7:27.99.