César Augusto Cielo Filho (Portuguese pronunciation: [ˈsɛzɐɾ si.ˈelu ˈfiʎu], born 10 January 1987) is a Brazilian competitive swimmer who specializes in sprint events. He is the most successful Brazilian swimmer in history, having obtained three Olympic medals, winning six individual World Championship gold medals and breaking two world records.

Cielo is the current world record holder in the 100-metre and 50-metre freestyle (long course). His gold medal at the 2008 Summer Olympics, in the 50-metre freestyle competition, is Brazil's only Olympic gold in swimming to date. In 2008, he broke the NCAA record in the 50-yard (46 m) freestyle (18.47 seconds) and in the 100-yard (91 m) freestyle (40.92 seconds). Cielo became the fastest swimmer in the world in the two distances, and was named NCAA Swimmer of the Year for the second year in a row.

César Cielo was born on January 10, 1987 in Santa Bárbara d'Oeste, São Paulo, Brazil. The son of pediatrician César Cielo and physical education teacher Flávia Cielo, Cielo began his athletic career at small swimming clubs in his native state. As a young teenager, Cielo trained under coach Mario Francisco Sobrinho at the Esporte Clube Barbarense, where his mother taught swimming. When he was 13, Cielo started training in Piracicaba at the Clube de Campo de Piracicaba under coach Reinaldo Rosa. At 16 years old, he transferred to Esporte Clube Pinheiros in São Paulo to train under coach Alberto Silva and Brazilian swimming legend Gustavo Borges. As a gift while at Esporte Clube Pinheiros, he received the swimsuit used by Borges in Athens 2004.

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