The men switched sides early on Monday in Musa Qala district, taking with them weapons and three motorcycles, a provincial spokesman told the BBC.
It is the second such defection in recent weeks – a police commander and 13 junior officers joined the Taliban in western Farah province in late July.
That was thought to be the biggest defection so far by the police.
It highlighted a major intelligence failure and was a propaganda boost for the insurgents, the BBC’s Bilal Sarwary in Kabul said.
Local police commanders are key to security and stability in remote areas where deal-making is crucial to compensate for a weak Afghan government.
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