Gausul Azam Dollar, 48, died at the Gaibandha Sadar Modern Hospital on Saturday morning, Jailer Md Masudar Rahman said.
The BNP leader, accused in a sabotage case, was arrested on Oct 5, Sadar police station Officer-in-Charge Mehedi Hasan said.
He had been kept at the district prison since Oct 6.
Jailer Rahman said Dollar was rushed to the hospital in the morning after he had fallen sick.
Sadar hospital's Medical Officer SIM Shahin said the BNP leader died from heart disease complications after being brought there.