Run by Jaroslaw Kaczynski, the twin brother of Poland's late president Lech, PiS secured 39.1 percent of the vote, well ahead of the centrist, staunchly pro-European Union PO on 23.4 percent, said pollster Ipsos.
A triumphant Kaczynski immediately declared victory. Such a score would give PiS 242 seats in the 460-member lower house of parliament, the exit poll showed, allowing the party to govern alone without the need for a coalition partner.
Prime Minister Ewa Kopacz of PO swiftly conceded defeat.
Poland has seen its economy, the largest in ex-communist central Europe, expand by nearly 50 percent in the last decade. But PiS was able to tap into widespread anger that the fruits of growth have not been evenly shared among the country's 38 million people.
Distrustful of the EU and an advocate of a strong NATO stance in dealing with Moscow, the party opposes joining the euro zone any time soon, promises more welfare spending on the poor and wants banks subject to new taxation.
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PiS has opposed relocating migrants from the Middle East to Poland, arguing they could threaten Poland's Catholic way of life. Kaczynski raised eyebrows this month by warning they could bring disease and parasites with them.
Michal Zurawski, in his mid-30s, who voted for PiS in the morning in central Warsaw, said he backed the party's promises to tackle corruption as well as its economic program.
"Their offer is targeted at those who are less affluent and that suits me. Taking care of this group and creating better social and labor conditions for them is good - it will benefit Poland's economy and the country as a whole," Zurawski said.
Pensioner Jadwiga Horus said she voted for PO: "I prefer people who have proven themselves. Even if they make mistakes - PO is way better than the shameful PiS."
On the campaign trail, Kaczynski and other PiS leaders sought to tap into anger that the economic success is not more evenly shared out and into nationalist sentiment fanned by immigration fears, particularly among young voters.
"There is a broader phenomenon of a return to national, religious, community values being seen all across Europe," said analyst Aleksander Smolar.
"PiS uses clear ... language in this respect."
PiS advocates a robust Western approach toward Russia, especially following Moscow's 2014 annexation of the Crimean peninsula in Poland's eastern neighbor Ukraine.
That might complicate any future attempts at bridge-building between the EU and Russia, currently the target of Western sanctions imposed over the Ukraine conflict.
