Abdul Rab Rassoul Sayyaf, who fought the Soviet occupation of Afghanistan in the 1980s and formed a close relationship with Osama Bin Laden, came fourth in the first round of voting on April 5.
His endorsement will likely bring Abdullah additional support from Pashtuns, Afghanistan's largest ethnic group.
Abdullah fell short of the 50 per cent threshold needed for an outright victory in the April first round and will face former World Bank economist Ashraf Ghani in the run-off.
"We have come together with our brothers who have the same mentality and background under the leadership of the Professor Sayyaf, to support our knowledgeable brother Dr Abdullah to save this country," Ismail Khan, a vice-presidential running mate of Sayyaf, told a gathering in Kabul.
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Despite being a former anti-Soviet resistance fighter, Abdullah is viewed with suspicion by many Pashtuns because he was a part of the Northern Alliance which opposed the Taliban during their 1996-2001 rule.
The militant group, who are predominantly Pashtun, have threatened to disrupt the run-off election, warning voters to stay away from polling stations for fear of injury or death.
Sayyaf is said to be the "mentor" of Khalid Sheikh Mohammed, the main plotter of the 2001 attacks on New York and Washington.
Sayyaf also ran militant training camps in Afghanistan and Pakistan throughout the 1980s and 1990s, which Mohammed attended and from which several of the 2002 Bali bombers graduated.
Sayyaf met Osama bin Laden at one camp and is said to have helped the Al-Qaeda chief return to Afghanistan in 1996.