Dadi and his son Ratnakar met YSRC honorary president Y S Vijaya at her residence and formally joined the party.
Earlier in the day, Dadi met YSR Congress president Y S Jaganmohan Reddy at the Chanchalguda jail and got his nod for joining the party, despite strong opposition from party leaders in Anakapalle, Dadi's hometown.
Dadi, who quit TDP after being a part of it for over three decades, said he would henceforth stand by the late Y S Rajasekhara Reddy's family.
"Now, I realised the truth and hence decided to walk with the late YSR's family," Dadi told reporters, coming out of the jail.
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Dadi, one of the senior leaders of TDP, has been an MLA for four terms and a minister twice.
He was, till May 2, Leader of Opposition in the AP Legislative Council for six years, ranking higher than the TDP chief Chandrababu Naidu in the protocol order.
His decision, however, has not gone down well with the TDP cadre in Anakapalle who stoutly opposed it.
Dadi and his son Ratnakar were virtually isolated as his entry into YSRC was also not welcomed by that party's local rank and file.
Former minister Konathala Ramakrishna, one of the very few leaders who have been a strong support for Jagan since he quit Congress in 2010, is a strong opponent of Dadi in Anakapalle. With Dadi's entry into the party, political equations for YSRC in Visakhapatnam district are expected to change.