Vinayagamurthi Muralitharan, alias Karuna, is already a government minister.
SLFP general secretary Maithripla Sirisena said Muralitharan was given the post held earlier by the late Anura Bandaranaike, brother of ex-president Chandrika Kumaratunga.
There are nine deputy chairmen in the party.
Sirisena said Muralitharan has been charged with increasing the membership of Tamil speaking people in the SLFP and strengthening the party in Tamil dominated areas.
The appointment comes as the remnants of the Tamil Tigers are making a last stand in a narrow coastal strip in north eastern Sri Lanka along with thousands of civilians held as a 'human shield'.
A top military commander said he believed Tiger chief Vellupillai Prabhakaran is still in the area and he may escape in a submarine.