Forty-four-year-old Khushboo joined the party in presence of Congress President Sonia Gandhi whom she met at her 10 Janpath residence.
She had quit M Karunanidhi's party in June saying her "hard work for the DMK had been a one-way path". She had joined DMK in 2010, when it was in power in Tamil Nadu.
Considered one of the finest actors in south Indian cinema, Khushboo has worked in over 100 films and shared screen space with superstars like Rajnikanth, Kamal Hassan, Mammootty and Mohanlal.
In 2010, the Supreme Court had cleared Khushboo of 22 cases filed against her for her remarks on pre-martial sex in an interview to a magazine in 2005.
It did not win a single seat in the Lok Sabha election in May. The party had also recently suffered another blow when its senior leader and former Union minister G K Vasan announced his own outfit.