Hashmi entered the plea Tuesday in Manhattan federal court to conspiracy to provide military support to al-Qaida.
Both sides in the case recommended a maximum prison term of 15 years for the plea.
The 30-year-old Hashmi had faced up to 70 years in prison if convicted of four criminal counts.
Jury selection was to set to begin in his trial on Wednesday.
Hashmi was arrested in 2006 in London and was brought to the United States. He has been held in solitary confinement in a federal lockup in Manhattan since 2007.