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.
Her joining the Congress is seen as a small boost for the party which has been reduced to a marginal player in Tamil Nadu politics.
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.


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Vaiko celebrates Prabhakaran's 60th birth anniversary
Chennai: NDA ally, MDMK chief Vaiko and pro-Tamil outfits on Wednesday celebrated the 60th birth anniversary of slain LTTE leader Velupillai Prabhakaran.
While Vaiko celebrated the birth anniversary of Prabhakaran at a village in Tirupur District, Nam Tamilar Katchi chief and film director Seeman cut a cake here.
Vaiko will also cut a cake in Erode at 7.30 pm,coinciding with the time of birth of Prabhakaran, partymen said.
Vocal pro-Tamil leader Pazha Nedumaran observed the anniversary of Prabhakaran, whose outfit is banned in the country, by planting a sapling at Thanjavur.
Other pro-Tamil leaders like Kolathur Mani have issued greetings messages on the occasion.
Police said several workers of MDMK were detained in Namakkal and Tirupur when they tried to celebrate the birthday in important road junctions of the town without permission.
Prabhakaran was killed by the Sri Lankan armed forces in 2009, ending decades of civil war in the island nation.