The unprecedented move comes after the government earlier this week formally asked President Andrzej Duda to impose such a measure for 30 days in parts of the Podlaskie and Lubelskie regions bordering Belarus.
"The situation on the border with Belarus is difficult and dangerous," Duda's spokesman, Błażej Spychalski, told a news conference on Thursday.
He added that his country was responsible not only for its own borders, but also those of the European Union as a whole.
"We must take measures to ensure the security of Poland and the European Union," he told reporters.
Poland began building a barbed-wire fence last week along the border in a bid to stem the flow of migrants from countries such as Iraq and Afghanistan crossing from Belarus.
Police stand guard on Poland's eastern border with Belarus amid a migrant crisis. Photo: PAP/Artur Reszko
The state of emergency is expected to give authorities broader powers to monitor and control the movement of people.
The Polish government spokesman on Thursday said the country was under "real threat" at its border with eastern neighbour Belarus.
"We are under real threat, both in terms of the migration flows orchestrated by" Belarus' President Alexander Lukashenko, "but also because of upcoming Russian-Belarusian military exercises, which pose a real danger," the spokesman, Piotr Müller, told state broadcaster TVP.
EU Commissioner for Home Affairs Ylva Johansson has said the standoff between Poland and Belarus "is not a migration issue, but part of the aggression" of Belarusian strongman Alexander Lukashenko "toward Poland, Lithuania and Latvia, with the aim to destabilize" the European Union.
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Source: TVP Info, PAP, Reuters