Y S Sharmila, the daughter of former Andhra Pradesh chief minister Y S Rajasekhara Reddy and founder of YSR Telangana Party, joined the Congress here on Thursday.
She also announced the merger of her YSR Telangana Congress with the Congress, and said she would fulfil any responsibility given to her.
Sharmila joined the Congress in the presence of party chief Mallikarjun Kharge and former party president Rahul Gandhi.
Lauding the Congress, she said it is the largest and "most secular" party in the country as it unwaveringly serves all communities and unites all sections of people.
"I am very happy to be merging the YSR Telangana party with the Congress. It gives me immense joy that the YSR Telangana party is going to be a part of the Indian National Congress from today onwards," she said.
Describing her father and former chief minister YSR Reddy as a legendary leader of the Telugu people, she said he not only served the Congress all his life, but also gave his life serving the Congress.
"Today, it would give him great joy that his daughter is following in his footsteps and is going to be a part of the Congress itself," she said.
"The Congress is still the largest secular party of our country. The Congress has always upheld the true culture of India. The Congress has built our nation's foundations. The Congress has unwaveringly served all communities, uniting all sections of the people of India," Sharmila said in the presence of top party leadership.
"I truly honour the Congress because it is the most secular party, the largest secular party of the country," she said.
She said Rahul Gandhi has won the confidence of the people with his Bharat Jodo Yatra and that also led to the party's victory in Karnataka.
Sharmila said she was happy that she was part of the Congress victory in Telangana. She had announced her support to the Congress during the recent assembly polls in Telangana to end the alleged corrupt and anti-people rule of the BRS led by K Chandrasekhar Rao.
"I also realised that is going to have a domino effect in Telangana and, therefore, I graciously withheld from contesting in Telangana. Congress stood a chance of victory in Telangana and we did not want to break the anti-KCR vote. The Congress won in Telangana and I am most happy that we contributed a part to that victory," she said.
She said it was her father's dream to see Rahul Gandhi as prime minister and she would be happy to contribute to it.
"It was my father's dream to see Rahul Gandhi as prime minister of our country and I am very glad that I am going to be a part in making that happen. Whatever responsibility the Congress will lay on me, I promise to fulfil that responsibility with all loyalty, integrity and diligence," Sharmila said.
She also raised the issue of ethnic violence in Manipur, and said "as a Christian it pained me that such cruelty took place in Manipur."
She said 2,000 churches were vandalised and 60,000 people became homeless in the state. "That kind of cruelty is something that I have not been able to digest even to this day. That struck me that this is what will happen if a secular party is not in power," she said.
"Therefore, I am joining the Congress and merging the YSR Telangana Party with the Congress very gladly today because I know that the Congress will uphold all sections of the people in Indian society," she asserted.
After chairing her party meeting on Tuesday in Hyderabad, Sharmila had said she and other leaders will meet the top leadership of the Congress, including Kharge, and make a "crucial" announcement in Delhi.
Sharmila is the younger sister of Andhra Pradesh Chief Minister Y S Jagan Mohan Reddy.