President Andrzej Duda signed the plan into effect after it was approved by both houses of parliament last month, state news agency PAP reported.
The law aims to protect Poland and the European Union against a migrant influx masterminded by Belarus, according to officials.
The government last month adopted a bill to construct a "permanent barrier" along Poland's eastern border with Belarus amid what officials said was a growing migrant surge.
The wall is expected to cost PLN 1.6 billion (EUR, 350 million, USD 410 million) and take months to build, according to the government.
Interior Minister Mariusz Kamiński said last month that the Polish border service "has been dealing with increasing migrant pressures" on the border with Belarus for several months.
"Despite the deployment of additional personnel and assistance from soldiers and police, as well as the use of temporary installations, we have not seen a decrease in the number of attempts to cross the Polish-Belarusian border," Kamiński said, as quoted by the PAP news agency.
"Therefore, urgent measures need to be taken to build a solid, high wall, equipped with a monitoring and movement detection system," he told reporters, adding that "such border security measures have worked for other countries and have proven to be an effective solution in the fight against illegal migration."
Defence Minister Mariusz Błaszczak has told public broadcaster Polish Radio that Poland plans to draw on the experience of Hungary, which constructed a permanent wall on its border with Serbia during a migrant crisis in 2015.
Poland and the Baltic states have accused Belarus's strongman leader Alexander Lukashenko of organising a wave of illegal migrants seeking to enter the bloc as part of what officials have called a "hybrid war."
The EU Commissioner for Home Affairs, Ylva Johansson, visited Poland in late September, agreeing with Warsaw’s arguments that “firm steps” were needed against Belarus, according to officials.
Poland's Prime Minister Mateusz Morawiecki said last month that his country enjoyed full support within the European Union as it worked to defend itself against a migrant influx and a "hybrid war" being waged by Belarus.
In late September, Polish lawmakers voted overwhelmingly to extend a state of emergency in parts of two regions along the country's eastern border with Belarus by two months amid a growing migrant surge.
The state of emergency gives authorities broader powers to monitor and control the movement of people on the Polish-Belarusian border, which is also the eastern border of the European Union.
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Source: IAR, PAP