He replaces A V Subramanian, who had held the post for about nine years.
Namassivayam represents Villianoor constituency in the Assembly. He has been legislator for the last 15 years.
He is related to the Chief Minister N Rangasamy, who is his political adversary.
After the announcement of his appointment as PCC leader by AICC, Namassivayam visited some temples and offered prayers at the presiding deities, sources said.
The outgoing PCC leader sent SMS messages to party cadres and media thanking them for cooperation during his tenure as PCC leader.