Congress's central observers - former Delhi chief minister Shelia Dikshit and general secretary Mukul Wasnik - announced the decision after holding discussions with the newly-elected party legislators here.
The Congress, which fought the May 16 elections in alliance with the DMK, won 15 seats in the 30 member assembly. The DMK won two seats.
Since there were three contenders for the chief minister's post - former chief minister V.Vaithilingam, party chief A.Namassivayam and Narayanasamy - there was a delay in choosing the next Chief Minister.
According to the party's central observers, Narayanasamy was elected unanimously by all the Congress legislators.
Meanwhile, disappointed at the outcome, supporters of Namassivayam shouted slogans and also attacked buses on the East Coast Road.
Narayanasamy, who is also a party general secretary, did not contest the elections. Now he has to get elected after one of the sitting legislators resigns.
The All India N.R. Congress (AINRC) won eight seats and the AIADMK grabbed four. An independent candidate was also elected.