A decree issued by President Jair Bolsonaro authorises the deployment of soldiers in nature reserves, indigenous lands and border areas in the region.
The announcement comes after intense pressure from European leaders.
Mr Bolsonaro has criticised the reaction of other nations, insisting wildfires "cannot be used as a pretext" for punitive sanctions.
It came after France and Ireland said they would not ratify a huge trade deal with South American nations unless Brazil does more to tackle blazes in the Amazon.
Many of the fires are thought to have been started deliberately, with suspicion falling on farmers who may benefit by having more available land.
Mr Bolsonaro has declared support for the clearing of areas of the Amazon for agriculture or mining and he has faced heavy criticism from experts and campaigners who say his administration has given a green light to rainforest destruction.
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Mr Bolsonaro also faces the possibility of international sanctions. Finland's finance minister has already called on the EU to consider banning Brazilian beef imports.
In a televised address to the nation on Friday, Mr Bolsonaro agued that forest fires "exist in the whole world" and "cannot serve as a pretext for possible international sanctions".
Environmental groups held protests in cities across Brazil on Friday to demand action to combat the fires, and protesters also gathered outside the Brazilian embassies around the world.
The largest rainforest in the world, the Amazon is a vital carbon store that slows down the pace of global warming. It is known as the "lungs of the world" and is home to about three million species of plants and animals, and one million indigenous people.
What are the new measures?
Mr Bolsonaro in his televised address confirmed that he had authorised the armed forces to help fight the fires. "I've learned as a military man to love the Amazon forest and I want to help protect it," he said.
The decree itself was fairly vague in its wording, but specified that the military would be deployed to nature reserves, indigenous lands and border areas in the region.
The deployment of soldiers would be left down to regional governors who can request "preventive action … against environmental crimes" and ask the army to "survey and combat fire outbreaks", it said.
Defence Minister Fernando Azevedo e Silva is to oversee the order and will be responsible for allocating resources, it stated. The order initially authorises action for a month, from 24 August to 24 September./BBC news