Alka Lamba, former Aam Aadmi Party MLA from Chandni Chowk, joined the Congress on Saturday in the presence of the party's Delhi unit incharge P C Chacko.
Lamba had announced her decision to join the Congress early last month when she resigned from the primary membership of the AAP.
Days after her announcement, she was disqualified as a member of the Delhi assembly by Speaker Ram Niwas Goel under the anti-defection law.
Lamba had quit the Congress after 20 years in December 2014 to join the AAP.
She was elected as MLA from Chandni Chowk assembly constituency on AAP ticket in 2015.
After her induction, Lamba said she decided to return to the Congress as it is the only party which can take care of Delhiites.
"The Aam Aadmi Party government led by Arvind Kejriwal did not do any concrete work for the benefit of Delhiites in the last five years. Though workers like me always put questions before him, they elicited no response from him," she said at the AICC headquarters in New Delhi.
Lamba alleged AAP supremo and Delhi chief minister Arvind Kejriwal made promises which he knew he would not be able to fulfil.
"Kejriwal, who functions like an autocrat, never listens to AAP workers and elected representatives of the party. As a result, development work has suffered. With an eye on the upcoming assembly elections, he is once again trying to fool the people of Delhi by making hollow promises," she said.
The Congress, led by Sonia and Rahul Gandhi, is the only party which can save the country from the present misrule, Lamba said, adding that the nation has suffered severe damage on all fronts in the last five years and slipped into a regressive mode.
Lamba's relationship with the AAP soured after she protested against a resolution in the Delhi Assembly in December last year demanding that Bharat Ratna awarded to former prime minister Rajiv Gandhi be withdrawn over the 1984 anti-Sikh riots.
She had been in touch with Congress leaders and had met party president Sonia Gandhi after announcing her decision to quit the ruling AAP.
The Congress could field Lamba from Chandni Chowk in the upcoming assembly polls, a senior leader of the party's Delhi unit said.
Amid talks of Lamba's return, four-time Congress MLA Parlad Singh Sawhney quit the party on October 6 and joined the AAP.
Sawhney was MLA from Chandni Chowk assembly constituency from 1998 to 2015.
In 2015, he was defeated by Lamba.