Witnesses and Amnesty International told The Associated Press the women were protesting on Monday along a major road in Adamawa's local government Lamurde when the soldiers shot at them after being blocked from passing.
Ten others were injured in the shooting, witnesses said.
The Nigerian Army in a statement denied killing anyone and blamed the deaths on a local militia it said opened fire in the area.
Amnesty International's Nigeria office said the agency confirmed soldiers killed the nine protesters, citing accounts from witnesses and families of victims.
"It shows that Nigerian military has not changed much because of its past record of human rights violations and disregard for rule of law," according to Isa Sanusi, director of Amnesty International in Nigeria.
The Associated Press could not independently verify what happened.
Such killings are common across Nigeria where soldiers often deployed in response to protests and clashes are usually accused of excessive use of force.
Protests against police brutality in Nigeria's economic hub of Lagos in 2020 ended up in what a government-commissioned inquiry described as a massacre after soldiers opened fire at the protest venue.
The latest incident happened amid a curfew that authorities imposed in Lamurde following frequent clashes between Adamawa's Bachama and Chobo ethnic groups over a prolonged land dispute.
The protesters were aggrieved that security forces, including the soldiers, were not enforcing the curfew in affected areas, thereby allowing the clashes to continue, according to Lawson Ignatius, the councilor representing Lamurde in the local government parliament.
Gyele Kennedy, who said his daughter was among the protesters shot dead, lamented in anguish that "we don't know what came over them".
"These soldiers were leaving where the conflict happened and they came to pass through this place. They came and met the women protesting when one of the soldiers shot his gun in the air. After that, they opened fire on the women," Kennedy said.
The Nigerian Army, however, denied the claims, saying its soldiers only engaged a local militia in a different part of the town.
"Without equivocation, the casualties were caused by the unprofessional handling of automatic weapons by the local militias who are not proficiently trained to handle such automatic weapons," an army spokesman said.
The reported killings come as the Nigerian military is under scrutiny from US President Donald Trump, who has alleged that Christians are being targeted in Nigeria's security crises and that the security forces are not doing enough to prevent the killings.
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