At a time when the Bharatiya Janata Party (BJP) is trying to gain a foothold in Tamil Nadu, its state unit Vice President BT Arasakumar joined MK Stalin's DMK.
Mr Arasakumar's decision to quit the party came after he had praised DMK President MK Stalin and said that the latter would become the Chief Minister of Tamil Nadu.
The BJP is in alliance with the ruling AIADMK in Tamil Nadu.
Mr Arasakumar joined the DMK in the presence of MK Stalin at the DMK headquarters.
Speaking to reporters, Mr Arasakumar said that many were sidelined in the BJP and those who had put in hard work for the party were not respected.
He had praised MK Stalin on Sunday at a wedding ceremony in Pudukottai district and said the DMK leader would soon become the Chief Minister of Tamil Nadu.