Jamie Paulin Ramirez, a US citizen and former resident of Colorado sometimes referred to as "Jihad Jamie", pleaded guilty to one count of conspiracy to provide material support to terrorists, the Justice Department said.
The 32-year-old faces up to 15 years in prison and a $250,000 fine at sentencing.
Officials alleged that Ramirez traveled with Colleen LaRose, who dubbed herself "Jihad Jane," to locations in Europe and elsewhere "to participate in and in support of violent jihad."
They were also said to have been involved in a plot to kill a Swedish cartoonist who made fun of the Prophet Muhammad.
LaRose, 47, pleaded guilty last month to related charges and could face life behind bars.
She was arrested in October 2009 for her role in a plot to kill Swedish cartoonist Lars Vilks, who unleashed a storm of protests from Muslims with his cartoons depicting the Prophet Muhammad seen as insulting to Islam.
LaRose sought to recruit men and women online, raise money and even agreed to carry out the murder of a Swedish resident, pledging "only death will stop me," the indictment charged.