The ruling All India Anna Dravida Munnetra Kazhagam on Saturday won the by-election to the Sattankulam assembly constituency in Tamil Nadu, officials said.
AIADMK's L Neelamegavarnam polled 57,385 votes while his Congress rival R Mahendran secured 41,930 votes.
The Tamil Maanila Congress, which merged with the Congress last year, had held the seat.
The election, held on February 26, was prompted by the death of sitting member Mani Nadar.
While the Dravida Munnetra Kazhagam had boycotted the poll, other opposition parties, including the Bharatiya Janata Party, preferred not to contest, leaving the AIADMK and Congress in a virtual straight battle.