Arvind Kashyap, a resident of Azamgarh, was arrested by the STF on a tip-off from Sarnath area of the district in connection with the murder case of BSP leader Sarvesh Singh Seepu, police said.
Kashyap was carrying a cash reward of Rs 5,000 on his head.
Over dozen criminal cases, including attempt to murder, were pending against Kashyap, they said.
Seepu, who was also a former SP MLA, and another person were shot dead by unidentified assailants on July 19 in Azamgarh.
Seepu's father Rampyare Singh was a cabinet minister in the previous Mulayam Singh Yadav government.