Tripura Chief Minister Manik Saha won the by-election from the Town Bardowali seat by a margin of 6,104 on Sunday, the Election Commission said.
In a close contest, the BJP won three of the four seats in the bypolls.
The chief minister, a BJP candidate, secured 17,181 votes, which is 51.63 per cent of the total votes polled.
His nearest rival Asish Kumar Saha of the Congress bagged 11,077 votes or 33.29 per cent of the total votes.
The Left Front was at the third spot as Forward Bloc candidate Raghunath Sarkar secured 3,376 votes (10.15 per cent).
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A Rajya Sabha MP, Saha was appointed the chief minister after a surprise step down by then chief minister Biplab Deb.
If already not a member of the Assembly, a newly-appointed chief minister is required to win a by-election within six months of his elevation to retain his seat.
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Saha will will now resign as an MP after being elected to the assembly, as per the rules.
The bypoll was held in the Town Bardowali seat after Asish Kumar Saha resigned as a BJP MLA and joined the Congress in February.
Over 78 per cent of Tripura’s 1,89,032 population had cast their votes in the bypolls which was contested by 22 candidates.
(With inputs from agencies)