Former Sar-e Pol provincial Sen. Qazi Abdul Hai served four years as a provincial chief starting in 2004. Abdul Ghafore Dastyaar, a deputy provincial governor, told the BBC the former senator defected but downplayed its significance.
"His defection does not have any impact on his people in the district, because he was not a very influential person," he said in a statement Thursday to the BBC.
The Taliban declared victory with the defection, saying it was a sign of their growing influence in northern Afghanistan.
Sar-e Pol, along with much of the Afghan north, has been relatively shielded from most of the violence in the country.
The BBC said the senator appears in a video posted on a Taliban website saying he's a former freedom fighter who witnessed "the corrupt face of the government" during his tenure.
International forces operating in Afghanistan are looking to wrap up their combat obligations next year as national forces take control.