Saiful Alam Nirob, former president of Jatiyatabadi Juba Dal, was released on bail on Friday after more than 15 months of imprisonment.
He walked out of the central jail in Keraniganj at about 3:00pm, said BNP senior joint secretary general Ruhul Kabir Rizvi.
Rizvi said that Nirob was granted bail in all pending cases against him, leading to his release from jail.
A group of BNP and Juba Dal leaders and workers welcomed Nirob with flowers in front of the prison gate.
Police had arrested Nirob in the capital on March 4 last year, when he was facing 454 cases.
He had been sentenced to 17 years in prison in seven cases prior to the 12th parliamentary election.