Final results of the weekend election announced on Monday showed Kaczynski gaining 54% of the vote, easily beating his pro-business rival, Donald Tusk of the Civic Platform.
A second round run off was needed after neither candidate scored the necessary 50% majority in the first round of voting two weeks earlier.
Kaczynski’s win is tightens the grip on power held by his party after it won general elections four weeks ago on a platform of pledges to shore up the welfare state and stamp out corruption.
The Law and Justice Party is headed by Kaczynski’s identical twin brother Jaroslaw.
Turnout for the second round presidential vote was relatively low at just under 51%.
Under the Polish constitution the presidency is largely responsible for foreign policy and defence issues and has less power than the country’s prime minister.
However, Kaczynski has said he intends to take on a more active and interventionist role in domestic affairs.
He comes to the presidency from his former job as mayor of Warsaw – a post where he has caused some raised eyebrows among Poland’s European Union neighbours for his strong views, not least his support for the death penalty.
In 2004 Poland was one of 10 mainly former Eastern Bloc states to join the EU, which makes abolition of capital punishment is a condition of membership.