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The pledge was spotted by SITE Intelligence, a jihadi monitoring group, on ISIS's messaging app of choice - Telegram.
In the Telgram channel titled Ansar al-Khilafah #Brazil, the Brazilian jihadis announced their dedication to ISIS leader Abu Bakr al-Baghdadi.
It comes less than three weeks before the 2016 Olympic Games are set to begin in Rio de Janeiro on August 5.
Meanwhile, France was last week informed of a planned terror attack on its team at the Games, according to the head of military intelligence.
The plans were revealed by General Christophe Gomart, head of the Directorate of Military Intelligence (DRM), at a parliamentary commission in May investigating the attacks in Paris in January and November last year which left 147 people dead.
In the report on the planned Rio attack, Gomart told the commission that he had been informed of the plot - planned by a Brazilian national - 'by our partners'.
No other details on the claims were made available by the French authorities.
Brazil's institutional security cabinet, which co-ordinates the information services in the country, told AFP they had received no information from the French authorities.
'We were not the source of information and the Brazilian intelligence agency (ABIN) was not officially informed either of this matter,' a spokesman told AFP.
In early July, the Brazilian Minister of Justice, Alexandre Moraes, said a jihadist attack in Rio de Janeiro during the Olympics was a 'possibility' but 'not a probability'.
Some 85,000 members of the security forces - 47,000 policemen and 38,000 soldiers - will be mobilized to ensure the safety of 10,500 athletes as well as officials, journalists and tourists from around the world expected for the Games which run from August 5-21.
Brazilian officials said on Friday that security preparations for the upcoming Olympics will be reviewed after the devastating attack in Nice, France.
Defense Minister Raul Jungmann told CBN radio in a Friday interview that safety protocols would be revised in the wake after Thursday's attack that killed scores of people leaving a fireworks display along a well-known boulevard.
A man drove a truck into a crowd of people celebrating Bastille Day in an attack that French officials called an undeniable act of terror. The killing only ended after police shot and killed the armed driver, who was identified as a 31-year-old Tunisian delivery man.
'The attack in Nice is worrisome for us, too. We have learned a few lessons. We will review procedures, make more barriers, searches and apply much tighter security,' Jungmann said.
'Unfortunately that can be an extra difficulty for people, but it is for their safety.'
Comment: Fear levels in this chaotic world seems to be at unprecedented levels. It looks like the coup in Brazil wasn't enough. Now they're pulling out the ISIS card...