S V Mohan Reddy, who represents Kurnool town, crossed over to TDP, 17th MLA in the state and fifth from Kurnool, a YSRC stronghold, to join the ruling party since February 22.
From 67, the YSRC's strength in the 176-member AP Legislative Assembly has now fallen to 50.
Chief Minister N Chandrababu Naidu, who was in Kurnool to take part in a water conservation programme today, formally inducted Mohan and other local leaders of YSRC into the party fold.
Mohan had announced last night that he was quitting YSRC "unable to stand the attitude" of Y S Jaganmohan Reddy.