Narayanasamy trounced his nearest AIADMK rival by 11,144 votes by securing 18,709 votes.
The by-election was held on Saturday following the resignation of Congress member John Kumar so that Narayanasamy could contest and continue as the Chief Minister.
Narayanasamy did not contest the assembly polls of May 2016. However, the Congress chose him as the Chief Minister.
Under law, a minister who is a non-member of an assembly has to get elected to the house within six months after assuming office.