ICG called on Islamabad to take action to thwart any plot targeting Asma Jilani Jahangir, a member of the ICG board and former president of the Supreme Court Bar Association of Pakistan.
Louise Arbour, ICG president and chief executive officer, said it would be "unconscionable" for Islamabad not to act.
"They must investigate the allegations immediately and thoroughly and bring to justice those who would make these kinds of threats against the life of this courageous woman," Arbour said in a statement from Brussels.
The International Crisis Group didn't indicate from where the threat originated.
U.N. High Commissioner for Human Rights Navi Pillay, following a weeklong visit to Pakistan, described the situation for women in Pakistan as "grim."
Human Rights Watch offered a similar assessment, saying reports of enforced disappearances and torture in Pakistan were widespread.
"This most recent threat against Asma is not only reprehensible and unacceptable, it is a challenge to the rule of law in Pakistan and a clear statement of disrespect for international human rights norms, which Asma has defended and advanced throughout her career," Tom Pickering, chairman of the ICG board, said in a statement.