Speaker V Vaithilingam administered oath to Narayanasamy, who took it in Tamil.
Narayanasamy won the Nellipthope Assembly bypoll, held on November 19, by defeating his immediate AIADMK rival Om Sakthi Segar by a margin of 11,144 votes.
Ministers, MLAs and leaders of the Congress, DMK and other alliance parties, and officials of various departments were among those who greeted him after the swearing-in ceremony.
With his induction, the strength of the Congress has been restored to 15 in the 30-member Assembly. DMK, which has two members, is also supporting the government.
69-year-old Narayanasamy was not a member of the Assembly when he assumed charge as Chief Minister on June 6.
A John Kumar of the Congress had vacated the seat on September 15 to enable Narayanasamy contest the bypoll before December-6 deadline for getting elected to the House.
Narayanasamy had served as Minister of State in the Prime Minister's Office in the second UPA government after serving as MoS Parliamentary Affairs in UPA-I.