Malik Agha's killing was the fifth in the past six weeks and came as violence mounts in the run up to the 18 September parliamentary elections that the Taliban have vowed to disrupt.
Agha was captured on Friday by Taliban members as he came out of a mosque in the Atghar district of Zabul and was hanged in a tree, district chief Gul Habib said on Saturday.
Agha was the chief of a powerful tribe in Zabul near the border with Pakistan, he said.
Abdul Latif Hakimi, a spokesman for the Taliban, said Agha was hanged because he was "a spy for the American forces".
Agha's death came a day after US forces killed 24 suspected Taliban and al-Qaida fighters in fierce fighting on the Afghan-Pakistan border.
On Friday, the fighters hit back elsewhere on the border, killing at least seven Afghan policemen and wounding five.
The fighters have vowed to kill anyone supporting President Hamid Karzai's Western-backed government and dozens of officials, soldiers, police and civilians have died in growing militant violence in the run up to the polls.
Fighters have vowed to kill
supporters of Hamid Karzai
Last Friday, suspected fighters stabbed to death a senior pro-government cleric and his wife in their home in the southeastern province of Paktika. Days later, they shot dead another cleric in southern Helmand province.
Another Ulema Council member was shot dead near the city of Kandahar on 3 July.
US-led forces overthrew the Taliban government after it refused to hand over al-Qaida chief Osama bin Laden, the alleged architect of the September 2001 attacks on the United States.