Reddy, a former MP, met Chief Minister N Chandrababu Naidu in the Secretariat here this evening and joined the TDP.
Incidentally, his brother Prabhakar Reddy quit the TDP in February this year and joined the YSRC. He immediately got elected as an MLC in March.
The Reddys head one faction in the Kurnool politics and wield a clout in their community in the region.
Silpa Reddy is now the YSRC nominee in the by-poll.
Speculation is rife that Pratap Reddy may now be given the MLC seat vacated by Silpa Chakrapani Reddy, who quit his post and joined the YSRC early this month.
The August 23 by-election is necessitated by the death of sitting MLA Bhuma Nagi Reddy on March 12.