The conservative but socially-oriented PiS received 43.6 percent of the vote and its main contender, the centrist Civic Coalition (KO), led by the Civic Platform (PO), received 27.4 percent.
The Left, a coalition of three left-wing parties, running under the Democratic Left Alliance (SLD) party banner, was supported by 11.9 percent.
The agrarian Polish People's Party (PSL), which ran together with the right-wing Kukiz'15 movement, was supported by 9.6 percent of voters.
The far-right Confederation was backed by 6.4 percent of voters in the Ipsos exit poll.
According to the poll, the United Right will get 239 mandates, KO can count on 130 mandates, SLD will take 43 seats, PSL will get 34 and Confederation will receive 13.
If the poll is confirmed by official results, the PiS-led coalition will again enjoy a ruling majority, which is at least 231 mandates in the 460-seat Sejm (lower house).
Voter turnout reached 61.1 percent, according to the survey. PiS won the elections in 14 provinces, while KO was victorious in two.
Ipsos ran the poll for the public TVP, and private TVN and Polsat TV stations.
PiS leader Jaroslaw Kaczynski, commenting on the provisional electoral results, said that despite the powerful front against PiS, the party had won, and the results were likely to remain unchanged.
Kaczynski said that PiS faces another four years in power, but also needs to reflect on where it was successful and why a part of society decided not to support the ruling party.
"In short, we are concluding a certain stage and we are opening a new one. And it is not going to be easier, but perhaps more difficult. I hope it will end in an even greater success," Kaczynski said.
The PiS leader vowed to further improve public finances and said the party's task was to better communicate its vision for a prosperous state.
PM Mateusz Morawiecki said on Twitter that Poles had decided they want the "good change" to continue.
"Poles decided today that they want to, first of all, continue the good change and then the good time for Poland."
He also added that, in anticipation of the final results, PiS would like to thank Poles for placing so much trust in it. "Our trademark is credibility, we will not let you down," Morawiecki said.
KO leader Grzegorz Schetyna said that "we will try to establish good cooperation in the Sejm and Senate, as cooperation among opposition groups is the only way. We are facing yet another election - presidential election, we believe in victory, there will be no Budapest in Warsaw."
Poland will hold the presidential ballot this coming spring.
SLD leader Wlodzimierz Czarzasty expressed joy with the return of left-wing parties to parliament.
"We promised our parties, we promised the left that we will return to the Sejm. It is a huge success, we have returned to the Sejm. We will be able to present our programme," Czarzasty said.
Left-wing parties were not represented in the previous parliamentary term.
PSL leader Wladyslaw Kosiniak-Kamysz said the voters trusted his party as a rational centrist grouping.
"There will be no dangerous right or left turns," Kosiniak-Kamysz said, but he also invited other groups to join his coalition.
Janusz Korwin-Mikke, a leader of Confederation, said various groups joined the project, including nationalists, ultra-Catholics, libertarians and monarchists. "We played as one team, we did great and I hope this will be confirmed in the results," he said.
Polish President Andrzej Duda congratulated all the MPs and senators who won mandates.
"Thank you to all my compatriots who today took part in the election. Congratulations to all the MPs and senators who won mandates, with such a high voter turnout. It's important! The voters did a great job!" Duda tweeted on Sunday.
The PKW plans to announce the first partial election results on Monday and final results on Tuesday. (PAP)