In February 2008, at the Missouri Grand Prix, Cielo improved his South American record in the 100-metre freestyle, with a time of 48.49 seconds.
In April, at the Ohio Grand Prix, he broke the record again, with a time of 48.34 seconds, also defeating Michael Phelps.

A month before the Olympics, in July 2008, he broke his South American record in the 50-metre freestyle, with a time of 21.75 seconds.

Afterward, Cielo went to the 2008 Summer Olympics in Beijing, where he broke the Americas record in the 4×100-metre freestyle heats, with a time of 47.91 seconds.
Although the Brazilian team was disqualified, Cielo's record was allowed to stand.
In the 100-metre freestyle, Cielo qualified for the final, taking eighth place in the heats, with a time of 48.07 seconds.
Cielo excelled in the early part of the first semifinal, making his turn in second place, at 22.62 seconds, but finishing fifth.
In the final, Cielo adopted a different tactic, turning third, slower (22.74 seconds) than in the semifinals, and saving energy for the finish, which resulted in his winning the bronze medal and breaking the South American record with a time of 47.67 seconds (tied with American swimmer Jason Lezak).
Alain Bernard won the gold medal and Eamon Sullivan won the silver.
Shortly after winning the bronze medal, Cielo gave a statement to the Brazilian press, stating categorically: "Now I'm going to win the 50m".
In the 50-metre freestyle event, Cielo set the Olympic record, that Alexander Popov had set in the 1992 Summer Olympics, during the heats (21.47 seconds, equaling Garrett Weber-Gale's Americas record) and in the semifinals (21.34 seconds).
He lowered this further in the finals, again breaking the Olympic record with a time of 21.30 seconds, winning the gold medal and missing the world record by 0.02 seconds.
With this, he became the first Brazilian Olympic swimming champion.
Until Cielo's Olympic gold, the best Brazilian Olympic swimming result had been obtained by Ricardo Prado, who won the silver medal in the 400-metre individual medley at the 1984 Summer Olympics, and previously Gustavo Borges, who won the silver medal in the 100-metre freestyle at the 1992 Summer Olympics and in the 200-metre freestyle at the 1996 Summer Olympics.

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.
In February 2008, at the Missouri Grand Prix, Cielo improved his South American record in the 100-metre freestyle, with a time of 48.49 seconds. In April, at the Ohio Grand Prix, he broke the record again, with a time of 48.34 seconds, also defeating Michael Phelps.
A month before the Olympics, in July 2008, he broke his South American record in the 50-metre freestyle, with a time of 21.75 seconds.
Afterward, Cielo went to the 2008 Summer Olympics in Beijing, where he broke the Americas record in the 4×100-metre freestyle heats, with a time of 47.91 seconds. Although the Brazilian team was disqualified, Cielo's record was allowed to stand. In the 100-metre freestyle, Cielo qualified for the final, taking eighth place in the heats, with a time of 48.07 seconds. Cielo excelled in the early part of the first semifinal, making his turn in second place, at 22.62 seconds, but finishing fifth. In the final, Cielo adopted a different tactic, turning third, slower (22.74 seconds) than in the semifinals, and saving energy for the finish, which resulted in his winning the bronze medal and breaking the South American record with a time of 47.67 seconds (tied with American swimmer Jason Lezak). Alain Bernard won the gold medal and Eamon Sullivan won the silver. Shortly after winning the bronze medal, Cielo gave a statement to the Brazilian press, stating categorically: "Now I'm going to win the 50m". In the 50-metre freestyle event, Cielo set the Olympic record, that Alexander Popov had set in the 1992 Summer Olympics, during the heats (21.47 seconds, equaling Garrett Weber-Gale's Americas record) and in the semifinals (21.34 seconds). He lowered this further in the finals, again breaking the Olympic record with a time of 21.30 seconds, winning the gold medal and missing the world record by 0.02 seconds. With this, he became the first Brazilian Olympic swimming champion. Until Cielo's Olympic gold, the best Brazilian Olympic swimming result had been obtained by Ricardo Prado, who won the silver medal in the 400-metre individual medley at the 1984 Summer Olympics, and previously Gustavo Borges, who won the silver medal in the 100-metre freestyle at the 1992 Summer Olympics and in the 200-metre freestyle at the 1996 Summer Olympics.
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.
César Cielo