R Selvaraj, who won the Neyyatinkara assembly segment in the June 4 by-poll as Congress candidate, was on Monday sworn in as MLA in the assembly.
Mr Selvaraj had resigned as CPI-M member of the assembly in March this year citing serious differences with the party leadership causing the by-election, before joining Congress.
He took oath in the name of God.
He won the seat by defeating his nearest rival CPI-M candidate F Lawrence by a margin of 6334 votes.
Mr Selvaraj had won the Neyyatinkara segment in 2011 assembly election.