Speaking to reporters here, Karunanidhi said the party has decided to contest and the candidate would be announced soon.
On Sunday, Karunanidhi said the party would study and take a decision on contesting the by-poll.
AIADMK's Mariam Pitchai, legislator from the constituency, died in a car accident in May hours before he was to be sworn-in as minister in Chennai.
In the 234-member assembly, the AIADMK has 149 members (including the speaker), the DMDK 29, the DMK 23, the Communist Party of India-Marxist 10, the Communist Party of
India nine, the Congress five, the PMK three, the MMK two, the PT two and the All India Forward Bloc one. One seat is vacant.