The opposition Civic Coalition, comprising the Civic Platform (PO) party formerly led by European Council President Donald Tusk and some smaller liberal parties, came second with 27.4% according to the Ipsos poll.
A late poll is based on partial results released by local electoral commision offices and is more precise than the exit poll.
An overall majority in the lower house of parliament would give PiS a strong mandate to continue its controversial reforms of the justice system, media, cultural institutions, banking and the energy sector.