Mohan George, who is the state committee member of Kerala Congress, a partner of the opposition United Democratic Front (UDF), will formally join BJP later in the day, party sources said.
George is a practicing lawyer at the Nilambur court and had earlier worked with different Kerala Congress factions.
George told reporters in Nilambur that he will present a good fight in the by-poll.
The BJP, which was initially hesitant to contest the by-poll, had left the seat to its NDA partner Bharat Dharma Jana Sena (BDJS).
As the party also shied away from contesting, the BJP contacted Mohan George and decided on his candidature.
The main contest in the by-poll, scheduled for June 19, is between CPI (M) state secretariat member M Swaraj, and the Congress nominee Aryadan Shoukath.
P V Anvar, who quit as MLA paving the way for the by-election, is also planning to contest after failing to reach an understanding with the UDF, his party sources said. PTI MVG MVG ROH
