Official results gave Komorowski 53.01 percent of the vote in Sunday's election to 46.99 percent for Jaroslaw Kaczynski, the BBC reported.
Kaczynski, the twin brother of Poland's last president, Lech Kaczynski, who died along with scores of other Polish officials in an April plane crash in Russia, had conceded defeat Sunday night.
Komorowski's victory brought a congratulatory call from U.S. President Barack Obama. The White House issued a statement saying the president told Komorowski "the resilience and resolve demonstrated by the people of Poland in the face of their unprecedented tragedy remains an inspiration to the world."
Obama also noted the close partnership between the United States and Poland in promoting "freedom, security and prosperity" around the world, the statement said, and thanked Komorowski for Poland's contributions in Afghanistan.
Obama invited the newly elected president to visit Washington.
The runoff was made necessary when no candidate garnered more than 50 percent in the first round of voting in June.
Komorowski is from the ruling Civic Platform party, while Kaczynski represents the conservative main opposition Law and Justice party.
Former lawmaker Urszula Gacek, also a member of the Civic Platform party, told the BBC many voters who had supported the left-wing candidate in the first round switched allegiance to Komorowski in the runoff.