Forty-three-year-old Ahmed Abu Khattala entered the plea through his lawyer today in a 15-minute court proceeding before a federal judge.
An 18-count grand jury indictment handed up last week makes Abu Khattala eligible for the death penalty if convicted.
The charges against him include murder of an internationally protected person, murder of an officer and employee of the United States and killing a person in the course of an attack on a federal facility.
Abu Khattala is the first militant to be prosecuted for the Sept 11, 2012 Benghazi attacks.