The 88-year-old veteran leader took oath before Speaker D Jayakumar. His party colleague and former Law Minister Durai Murugan also took oath.
The two leaders had not taken oath on May 23, when the first session of the House convened under Pro-Tem speaker C K Thamizharasan for oath-taking as they had left for Delhi to meet Karunanidhi's jailed daughter Kanimozhi.
Kanimozhi is in Tihar jail in Delhi on bribery charges in connection with the 2G spectrum allocation scam.
Asked how his party would perform in the Assembly, Karunanidhi said, "observe closely".
Till recently in the saddle of power, the DMK was unseated by the J Jayalalithaa-led AIADMK following the latter's spectacular victory in the April 13 Assembly polls.
In fact, the ruling party was relegated to a tame third spot in the Assembly by virtue of scoring fewer number of seats than actor-politician Vijayakant's DMDK. DMDK is now the Principal Opposition party and Vijayakant the Opposition Leader.