Abdullah pulled out of a second-round run-off in 2009 after collecting about 30 percent of the vote, allowing President Hamid Karzai to retain power in an election that was badly marred by cheating and violence.
"We will make sure that the people of Afghanistan have a fair election," Abdullah said as he arrived at the Independent Election Commission (IEC) offices in Kabul and registered for the April 5 poll.
International donors are pushing hard for a credible election process next year after billions of dollars of development aid and 13 years of bloody fighting by US-led NATO forces against Taliban militants.
Abdullah, a former eye surgeon, has remained bitter about his old rival Karzai since the 2009 poll. He has previously alleged that the president, who is constitutionally barred from standing for a third term, is plotting to extend his time in office.
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Abdullah was a close aide of Ahmad Shah Massoud, the anti-Taliban commander who was assassinated in 2001, and his fluent English and assured manner have earned him a strong reputation in international political circles.
Abdullah, son of a Pashtun father from Kandahar and a Tajik mother, still lacks a loyal voter base. But he could build up a broad spectrum of allies and appears to have calculated that he has a serious shot at the presidency.
Now aged 53, he remains in favour with many Western diplomats who admired his energetic campaigning and dignified conduct during the last election.
In 2009 the Electoral Complaints Commission threw out around a third of the votes -- about half a million -- cast for Karzai, sparking the run-off from which Abdullah ultimately withdrew "in the best interests of the nation".
Ghani, who has not yet registered with the IEC, said on Monday he would contest the election despite finishing a distant fourth in 2009.
Hectic political deal-making has been underway for weeks in Kabul, but it remains unclear who else will register.
Karzai's brother Qayum is tipped to enter, as is low-key Foreign Minister Zalmai Rassoul.
President Karzai has repeatedly said that he sees a free and fair election as a key part of his legacy, and he has vowed not to endorse any candidate as Afghanistan prepares for its first ever democratic transfer of power.