The conservative New Frontier Party (NFP) on Wednesday won 152 seats in the 300-seat National Assembly, 25 more than the opposition Democratic United Party (DUP), which was tipped to score an easy victory in earlier opinion polls.
"People made wise choices," President Lee Myung-Bak said in a statement.
"The government will do its best to manage state affairs in a stable manner and take care of the people's livelihood," Lee, who cannot constitutionally stand for a second term, said.
The ruling party had initially struggled to preserve its parliamentary majority to pave the way for a second successive presidential victory. It had 165 seats in the outgoing parliament against 89 for the DUP.
"The DUP failed to turn public calls for punishing the... ruling party into reality. We apologise for disappointing (supporters)," DUP secretary general Park Sun-Sook told reporters, effectively conceding defeat.
"We will sincerely think over what today's election means and try ceaselessly to be reborn as a party the people can lean and rely on."
The leftist opposition Unified Progressive Party took 13 seats while five seats went to the right-wing Liberty Forward Party.
Presidential vote bellwether
The election is largely seen as a bellwether for the presidential vote in eight months' time.
The outcome is expected to bolster the position of NFP leader Park Geun-Hye, a presidential hopeful. She has tried to rebuild the NFP since she took over last December.
The ruling party ditched its old name of the Grand National Party and moved to the left to try to shake off its image as a party for the rich. It pledged to improve state welfare programmes.
Analysts said regionalism had weighed heavily in the election.
The NFP almost swept Park's regional base of southeastern Gyeongsang provinces while DUP candidates were outstanding at their home base of southwestern Jeolla provinces.
Turnout was 54.3 per cent compared to 46.1 per cent four years ago.
With economic concerns sidelining worries over North Korea, the DUP tried to exploit discontent over rising prices, high education and housing costs, job difficulties, a widening income gap and a weak welfare system.
The ruling party depicted its opponents as socially divisive and bent on undermining a decades-old security alliance with the United States, particularly through their vow to renegotiate a recently ratified free trade deal with the US.
North Korea's impending rocket launch is the focus of international attention but has barely figured in the election campaign in the south, which is used to tension with its communist neighbour.
Pyongyang, nevertheless, has repeatedly urged South Koreans to vote out the conservatives who scrapped a cross-border aid and engagement policy.
"Young voters, students and people must deliver a crushing defeat to the traitors," the ruling party daily Rodong Sinmun said.
The parliament has 246 directly contested seats and 54 proportional representation seats, allocated according to the total number of votes parties receive nationwide.
