Tirath (59) became the third prominent personality in the recent days to join BJP after Kiran Bedi and Shazia Ilmi.
Her joining BJP comes amidst the party's efforts to woo Dalit voters of the capital to boost its prospects in the February 7 polls.
Delhi has 12 reserved seats of which a majority were won by the AAP in the last assembly polls in 2013. BJP won only two while Congress got one.
Tirath, who had served as a minister in Delhi government and later as a union minister in the Manmohan Singh government, was welcomed in the BJP fold by its chief Amit Shah with a bouquet of flowers.
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She was accompanied by incharge of party affairs in Delhi Prabhat Jha and city unit BJP president Satish Upadhyay.
Explaining why she was switching the party, Tirath said, "It is a question of ideology. I met BJP President Amit Shah to serve the people.... I have come with this aim."
On what would be her role in the BJP, she said the party chief will decide that and added, "My role will be towards public service."
Congress appeared to be taken aback by Tirath's move to desert the party. Reports of her meeting Shah to join BJP reached AICC headquarters when AICC general secretary Ajay Maken was releasing a booklet "exposing" Aam Aadmi Chief Arvind Kejriwal.
A senior Congress leader said on the condition of anonymity that Tirath was trying hard for a ticket from the Patel Nagar constituency but was denied the ticket by party.