The declaration will free the authorities to allocate "infrastructure funds and to put in place humanitarian measures" on the Venezuelan border, the government said in a statement.
In addition, "military assets will be doubled to 200 soldiers," said Defence Minister Raul Jungmann said. A decree should be signed in the next few days, the statement said.
The Brazilian government says that some 40,000 Venezuelans have now fled the economic breakdown and political unrest in their oil-rich country to take shelter in Boa Vista, capital of Brazil's border state of Roraima.
Last week, three Venezuelans were hurt in an arson attack on a residence housing 13 migrants.