"My commitment to my profession is more than my commitment to my faith...It is the highest form of faith for me," he told reporters here.
Jethmalani said he would be appearing for Sen, who has challenged his sentence in the Chhattisgarh High Court, free of cost. "I am doing it for free," the Rajya Sabha MP said.
The BJP had earlier said that it saw nothing wrong in Jethamalani appearing for Sen.
61-year-old Sen, a paediatrician by profession, was convicted for sedition and sentenced to life in prison along with Naxal ideologue Narayan Sanyal and Kolkata businessman Piyush Guha for colluding with Maoists to establish a network to fight the state.