Rajesh Kumar (34), a drug peddler, was arrested recently. He was sent to judicial custody on April 20.
Last night, he complained about unrest following which the jail authorities shifted him to the civil hospital where he was declared brought dead, the officials said.
Kumar's body was handed over to his kin today after a postmortem.
According to the officials, a judicial inquiry had been ordered to ascertain the exact cause of the undertrial's death.