On April 25, 2012, participating in the Maria Lenk Trophy in Rio de Janeiro, Cielo finished the 50-metre freestyle race in 21.38 seconds, earning the best time of the year in the event, and getting close to the best time in the event's history without using super-suits (21.36 seconds made by Frédérick Bousquet).
A day later, he broke the Americas record for the 50-metre butterfly, with a time of 22.76 seconds beating the previous record, 22.87 seconds set by Nicholas Santos.
It was a world textile best.

At the 2012 Summer Olympics in London, Cielo finished 6th in the 100-metre freestyle, with a time of 47.92 seconds.
The gold went to Nathan Adrian (47.52 seconds), followed by James Magnussen (47.53 seconds) and Brent Hayden (47.80 seconds).
At the 50-metre mark, Cielo had the fastest split with a time of 22.60 seconds.
In the 50-metre freestyle, where Cielo was the favorite to win gold, he ranked first in the semifinal, tied with Cullen Jones, with a time of 21.54 seconds.
But in the final, he finished with a worse time than in the semifinal round, but managed to earn his third Olympic medal, a bronze, with a time of 21.59 seconds.
The winner was Frenchman Florent Manaudou with a time of 21.34 seconds.
Cielo's Olympic record of 21.30 seconds set in 2008 still held.

On August 20, 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.

At the end of the year, Cielo had an operation on both knees.
Since 2007, the Brazilian had suffered patellar tendinopathy in both knees.
Cielo was in constant pain, and the injury started to hurt his training and performance.
The nadir was at the 2012 London Olympics, where Cielo was already struggling at the start.
In September, he had an operation on the patellar tendon. "
He had a chronic wear of the patellar tendon, which generated an inflammatory process that bothered him greatly.
In London, Cielo has lost much of the leg muscles because this inflammatory process.
He came to lose two inches in circumference on each leg for a period of ten days.
As the inflamed knee, he lost output quality.
An output was once the best in the world ended up much like those of their adversaries" said Gustavo Magliocca, Cielo's doctor.
On April 25, 2012, participating in the Maria Lenk Trophy in Rio de Janeiro, Cielo finished the 50-metre freestyle race in 21.38 seconds, earning the best time of the year in the event, and getting close to the best time in the event's history without using super-suits (21.36 seconds made by Frédérick Bousquet). A day later, he broke the Americas record for the 50-metre butterfly, with a time of 22.76 seconds beating the previous record, 22.87 seconds set by Nicholas Santos. It was a world textile best.
At the 2012 Summer Olympics in London, Cielo finished 6th in the 100-metre freestyle, with a time of 47.92 seconds. The gold went to Nathan Adrian (47.52 seconds), followed by James Magnussen (47.53 seconds) and Brent Hayden (47.80 seconds). At the 50-metre mark, Cielo had the fastest split with a time of 22.60 seconds. In the 50-metre freestyle, where Cielo was the favorite to win gold, he ranked first in the semifinal, tied with Cullen Jones, with a time of 21.54 seconds. But in the final, he finished with a worse time than in the semifinal round, but managed to earn his third Olympic medal, a bronze, with a time of 21.59 seconds. The winner was Frenchman Florent Manaudou with a time of 21.34 seconds. Cielo's Olympic record of 21.30 seconds set in 2008 still held.
On August 20, 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.
At the end of the year, Cielo had an operation on both knees. Since 2007, the Brazilian had suffered patellar tendinopathy in both knees. Cielo was in constant pain, and the injury started to hurt his training and performance. The nadir was at the 2012 London Olympics, where Cielo was already struggling at the start. In September, he had an operation on the patellar tendon. "He had a chronic wear of the patellar tendon, which generated an inflammatory process that bothered him greatly. In London, Cielo has lost much of the leg muscles because this inflammatory process. He came to lose two inches in circumference on each leg for a period of ten days. As the inflamed knee, he lost output quality. An output was once the best in the world ended up much like those of their adversaries" said Gustavo Magliocca, Cielo's doctor.
César Cielo