She had quit the Congress in 2014 to be a part of Arvind Kejriwal's Aam Aadmi Party (AAP) but returned to the Congress after five years.
"I am very happy that the Congress party has again accepted me as its member. Though I was away from the party, I could not get separated from its ideology," she told ANI.
"I am thankful to Sonia ji and Rahul ji and also other Congress leaders. I have joined the party to give voice to the people and their issues. I have joined the party to give more power to it," added Lamba.
Lamba was disqualified from the legislative assembly in September on the ground of defection. The order of her disqualification was issued by Delhi Assembly Speaker Ram Niwas Goel days after she announced joining the Congress.
Speaker Goel passed the order of disqualifying her from the House after considering a petition moved by AAP MLA Saurabh Bharadwaj. Lamba represented Chandni Chowk Assembly seat of the national capital.