Sunday's run-off election pitted Kaczynski and his Law and Justice Party against Donald Tusk of the Civic Platform party.
The Polish public television channel TVP1 said Kaczynski won 52.8 percent and Tusk, 47.2 percent.
The International Herald Tribune said turnout was only 50.6 percent, which spokesmen in both camps called extremely disappointing for a country that won its democracy only 15 years ago.
On domestic policy, Kaczynski, the mayor of Warsaaw, has promised to cut bureaucracy and end corruption, and increase family benefits and pensions.
Speaking to a rally after Tusk conceded, 56-year-old Kaczynski referred to earlier remarks about placing restrictions on communists, homosexuals and other minorities.
"Poland needs to reconsider its mistakes," Kaczynski told the crowd. "But more than that, it needs a consensus based on truth."