The party's election campaign included promises to increase welfare spending, lower retirement ages and oppose EU refugee quotas
Polish Prime Minister Ewa Kopacz has conceded defeat as exit polls suggest the right-wing Law and Justice party may achieve an overall majority in parliament.
The exit poll by Ipsos predicts the party will win 39.1% of the popular vote, enough to secure 242 seats in the 460-seat house, which would allow it to govern alone.
It marks a significant lurch to the right by the Polish electorate - Law and Justice is socially conservative and midly euroskeptic, and has boosted its popularity with a harsh stance on immigration.
Its election campaign included promises to increase welfare spending, lower retirement ages and oppose EU refugee quotas.
It would be the best ever showing for the party, who have not been in government since 2007.
AP reports incumbents Civic Platform are expected to take 133 seats, with the rest going to Kukiz'15, led by musician Paweł Kukiz, the pro-business Modern Poland and the Polish Peasants Party.
Official results are expected on Monday.