César Augusto Cielo Filho (Portuguese pronunciation: [ˈsɛzɐʁ siˈelu ˈfiʎu], born 10 January 1987) is a Brazilian former competitive swimmer who specialized in sprint events.

As a young teenager, Cielo trained under coach Mario Francisco Sobrinho at the Esporte Clube Barbarense, where his mother taught swimming.

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.

Cielo was so eager for victory that during the 1996 Summer Olympics, at nine years old, he was already studying his main reference, Russian Alexander Popov, through videos, noting details like his starts (block outputs) and turnarounds.

[11] On a team with Guilherme Guido, Kaio Almeida and Eduardo Fischer,[12] he finished 4th in the 4×100-metre medley,[13] breaking the South American record with a time of 3:33.02.

[18] He also competed in the 4×200-metre freestyle,[19] where he broke the South American record with a time of 7:06:09, along with Rodrigo Castro, Thiago Pereira and Lucas Salatta.

[20] In December, he began to stand out on the national scene, by breaking the South American record of Fernando Scherer in the 100-metre freestyle (48.69 seconds) which had stood since 1998.

[29] At that time, Cielo had established himself as the fastest Brazilian sprinter, by breaking the Fernando Scherer's South American record in the 50-metre freestyle.

He also broke the Pan Am Games and South American record in the 4×100-metre freestyle, with a time of 3:15.90, along with Fernando Silva, Eduardo Deboni and Nicolas Oliveira.

After the Olympics, in October, in the first stage of the 2008 FINA Swimming World Cup, held in Belo Horizonte, Brazil, Cielo equaled the short-course South American record in the 50-metre freestyle, with a time of 21.32 seconds.

[54][55] At the 2009 World Aquatics Championships in Rome, Cielo led the Brazilian 4×100-metre freestyle relay team to a 4th-place finish, along with Nicolas Oliveira, Guilherme Roth and Fernando Silva.

[63] When he finished the 4×100-metre medley, a contest where the first four relays in the race beat the US world record from the 2008 Summer Olympics, Cielo led Brazil to fourth place, along with Guilherme Guido, Henrique Barbosa and Gabriel Mangabeira, very close to winning the event's bronze and silver medals.

[64][65] Cielo became the sixth Brazilian to achieve a world record in the long course, after Maria Lenk, Manuel dos Santos, José Fiolo, Ricardo Prado and Felipe França.

[67] On 18 December 2009, in São Paulo, Cielo broke the world record in the 50-metre freestyle, with a time of 20.91 seconds, at a championship hosted at Esporte Clube Pinheiros, his club in Brazil and where he had trained since 2003.

He began the year receiving proposals from several teams in Brazil and elsewhere, but he chose to sign with Flamengo, under the chairmanship of former swimmer Patrícia Amorim, with a mission to contribute to the strengthening of swimming in Rio de Janeiro and other big Brazilian clubs.

[76][77] At the December 2010 FINA Short Course World Swimming Championships in Dubai, Cielo, along with Nicholas Santos, Nicolas Oliveira and Marcelo Chierighini, won the bronze medal in the 4×100-metre freestyle, with a time of 3:05.74, setting a South American record and leaving behind the US team.

[83][84] Completing his participation in the Short Course Worlds, Cielo led the Brazilian 4×100-metre medley relay team of Guilherme Guido, Felipe França Silva and Kaio de Almeida to win the bronze medal.

The initiative, designed to serve as Brazilian swimming's "elite squad", gathered Cielo, who would be leading the project, and six other swimmers – André Schultz, Leonardo de Deus, Nicholas Santos, Henrique Rodrigues, Tales Cerdeira and Vinicius Waked – all selected for potential medals in the 2016 Summer Olympics, in Rio de Janeiro.

[105] For two years, the innovative program, which proposed a system of high level training without relying on the infrastructure of big clubs, worked.

[113] On 20 August, at the Jose Finkel Trophy in São Paulo, Cielo broke the short-course South American record in the 4×50-metre freestyle, with a time of 1:25.28.

[122] Only four male swimmers have won more gold individual medals in World Championships than Cielo: Aaron Peirsol (7), Grant Hackett (7), Ryan Lochte (9) and Michael Phelps (15).

[123] Cielo's results were considered a "redemption", because of low expectations entering the 2013 World Championships due to the troubled period following the London Olympics, when he underwent surgery on both knees, left his club and changed his coach.

[124] On 23 April 2014, participating in the Maria Lenk Trophy competition in São Paulo, Cielo finished the 50-metre freestyle race in 21.39 seconds, earning the best time of the year in the event.

[127] On 3 September 2014, participating in the José Finkel Trophy (short course) competition in Guaratinguetá, Cielo finished the 50-metre freestyle race in 20.68 seconds, earning the best time of the year in the event.

[135] Cielo also closed the competition with a flourish, leading the Brazilian quartet to an unprecedented gold in the traditional Men's 4 × 100 metre medley relay, with a time of 3:21.14, South American record.

[136] Cielo didn't participate in the 2015 Pan American Games in Toronto, Ontario, Canada, targeting the World Championships, who would happen after some days.

At the 2015 World Aquatics Championships in Kazan, Brazil finished 4th in the Men's 4 × 100 metre freestyle, in a relay composed by Bruno Fratus, Marcelo Chierighini, Matheus Santana and João de Lucca.

[143] In April 2016, at the Maria Lenk Trophy, held in Rio de Janeiro, Cielo failed to qualify for the 2016 Summer Olympics.

[149] At the 2018 FINA World Swimming Championships (25 m) in Hangzhou, China, Cielo, along with Marcelo Chierighini, Matheus Santana and Breno Correia, won the bronze medal in the Men's 4 × 100 metre freestyle relay, with a time of 3:05.15, setting a South American record.

[150][151] He won another bronze medal in the Men's 4 × 50 metre medley relay, along with Guilherme Guido, Felipe Lima and Nicholas Santos.