Dravida Munnetra Kazhagam (DMK) leader MK Stalin was elected as the party's working president in the general council meeting on Wednesday.
The meeting - earlier scheduled for 20 December - had to be cancelled in view of party president M Karunanidhi's hospitalisation.
The meeting was chaired by party general secretary K Anbalagan.
Five times chief minister of Tamil Nadu, Karunanidhi had chosen Stalin as his heir four years ago.
Stalin, also Treasurer of the DMK was born to Karunanidhi's second wife, Dayalu Ammal.
"This is a huge responsibility. I accept the appointment as Working President," Stalin was quoted by NDTV.
With Stalin's promotion as the working party president, Tamil Nadu's other major political force has seen a significant transfer of power too.
Stalin's election takes place amid rival All India Anna Dravida Munnetra Kazhagam (AIADMK) leaders' appeal to VK Sasikala to take over as Tamil Nadu chief minister.
Senior AIADMK leader M Thambidurai had appealed to the the newly-appointed party general secretary earlier this week to assume the chief minister's post.