"I have joined the BJP today, I am very much confident. I believe BJP is not a communal force but a secular force," Mr Roy said, encouraged to speak in Bangla by Union Minister and senior BJP leader Ravi Shankar Prasad, who announced his induction.
Mukul Roy, 63, said he is confident that without having the support of the BJP, the Trinamool Congress could not have got the kind of success it had. The two parties have been allies in the past.
Mr Roy, once West Bengal Chief Minister Mamata Banerjee's number two in the party, quit the Trinamool Congress and resigned his Rajya Sabha seat last month, saying a "lack of principle" had impelled his exit from a party he helped launch.
Amid speculation that Mr Roy was headed to the BJP, Kailash Vijayvargiya, the party's leader in charge of Bengal, said last week that the former Trinamool man had expressed to the BJP's West Bengal unit, a wish to join the party.
Mr Roy was called Chanakya in the Trinamool Congress for his organisational and strategic skills. The party promptly suspended him for six years, accusing him of indulging in anti-party activities after he announced that he was quitting.
His fallout with Mamata Banerjee has been linked to the rise in the party of her nephew Abhishek Banerjee, who replaced Mr Roy's men with his own as the second rung in the Trinamool.
The Trinamool has not said a word on Mr Roy's switch to the BJP. But his most clichéd answer at the press meet in Delhi could be a cause of concern for West Bengal's ruling party. As a former general secretary, Mr Roy would know of most skeletons in the Trinamool's cupboards.
Asked about his links to the Narada sting case, the new BJP leader said, "The law will take its own course."
The law has already led to the dozen Trinamool leaders named in the Narada chargesheet being summoned to ED or CBI, including Mr Roy. Of 12 leaders seen on the sting tapes, Mr Roy is the only one not seen accepting cash.