The ruling Saenuri Party won both National Assembly seats up for grabs in Wednesday's by-elections. The two were for a constituency in Hwaseong, Gyeonggi Province south of Seoul, and another in the southeastern port city of Pohang, a ruling-party stronghold.
The latest election brings to four out of five the by-election wins of the ruling party since President Park Geun-hye took office in February.
The only district the ruling party lost was to software tycoon and independent Ahn Cheol-soo, who won Nowon in northeastern Seoul in April.
In Hwaseong, Saenuri candidate Suh Chung-won won 64.4 percent of ballots against 27.6 percent for rival Oh Il-yong of the main opposition Democratic Party.
In Pohang, ruling-party candidate Park Myung-jae crushed opposition rival Heo Dai-man with 79.3 percent against 18.5 percent.
A Saenuri official attributed the wins to the opposition's excessive milking of a scandal around the National Intelligence Service's attempts to influence the presidential election in favor of Park. The DP said it would "humbly" accept the decision of voters.
But voter turnout was paltry at 33.5 percent, the lowest since 31 percent in by-elections in April 2007.