Mitra, Editor and Managing Director of The Pioneer newspaper, quit the Bharatiya Janata Party earlier this week.
Trinamool Congress secretary general Partha Chatterjee announced Mitra's switch to the Trinamool from the dais of the hugely attended rally in central Kolkata.
Mitra was nominated as a member of the Rajya Sabha in August 2003. He was elected for another term in the Upper House as a BJP MP from Madhya Pradesh in June 2010.
He unsuccessfully contested the 2014 Lok Sabha elections on the BJP ticket from Hooghly constituency in West Bengal.
Besides Mitra, four state Congress legislators - Samar Mukherjee, Abu Taher, Sabina Yasmin and Akhruzzman -- joined the Trinamool. They are among a growing band of Congressmen who have crossed over to the Trinamool Congress but abide by the parent party's whip in the state assembly to avoid attacting the anti-defection law.
Former CPI-M MP Moinul Hassan, who was recently expelled after he publicly dissociated from it, also joined the Trinamool at the rally.