"I would do it for him, I would fight for Binayak Sen. No party line says a lawyer should not fight a case," Jethmalani told CNN-IBN.
He stressed that the case was based on weak evidence.
"I have not seen the judgment, but on the basis of the case that was filed I can say it was a case of low evidence," he added.
Jethmalani, a Rajya Sabha MP from BJP, earlier appeared for Sen in the Supreme Court when he was seeking bail. He argued against the state government following which Sen was granted bail.
The BJP, which is the ruling party in Chhattisgarh where Sen has been convicted by a court, has been maintaining a hard stance against the Maoists. It believes Sen is a Maoist sympathiser.