Prime Minister Donald Tusk has become the first Polish leader to be re-elected since the end of communism.
Officials results, announced after 93% of votes had been counted, gave Mr Tusk's Civic Platform party enough seats to continue in coalition, the BBC reports.
The centre-right Civic Platform took 39% of the vote, against 30% for its conservative challenger, the Law and Justice Party.
The leader of Law and Justice, Jaroslaw Kaczynski, has admitted defeat.
Civic Platform becomes the first Polish party to win two consecutive terms since communism's fall in 1989.
Mr Tusk, 54, appears to have been rewarded for presiding over four years of strong economic growth, since winning a snap election in 2007.
Poland has been the only European Union member state to avoid recession, and this year its economy is forecast to grow by about 4% - the highest rate among the EU's seven largest economies.