The human rights group's new approach overturns a previous neutral position on the debate over abortion and women's rights.
The new policy forms part of Amnesty's global campaign to stop violence against women.
Widney Brown, senior policy and campaigns director, says Amnesty also viewed abortion as a right for women whose health was threatened by a pregnancy and that the group would call for the procedure to be decriminalised globally.
She said the board of the London-based group agreed on the policy last month after two years of consultations involving experts and the organisations more than 2.2 million members.
But they face opposition to the policy change as more than 70 members of a bipartisan US Congressional caucus against abortion urged Amnesty to oppose the surgical procedure.
The US Conference of Catholic Bishops also criticised the decision, saying Amnesty risked its "excellent record as a champion of human rights" if it abandoned its former neutral stance.
Abortion is one of the most divisive issues in the United States, where opponents campaign relentlessly to overturn a 1973 Supreme Court ruling that women have a basic constitutional right to abortion.
Ms Brown made it clear Amnesty International was not backing abortion as a "fundamental right" for women because that approach was not supported by international human rights laws.
World Health Organization data shows that some 45 million unintended pregnancies are terminated annually, of which an estimated 19 million are carried out unsafely.
It said an estimated 68,000 women die every year from an unsafe abortion.