Colleen LaRose, 47, helped foreign terror suspects intent on starting a holy war in Europe and South Asia, prosecutors said.
LaRose, who also was accused of using the online screen name "Fatima LaRose," has been in custody since October 2009 and faced a possible life sentence under charges in a four-count indictment.
Speaking clearly but quietly, LaRose said on Tuesday she had never been treated for any mental health problems and was entering her plea freely. She whispered a few comments to her lawyers, some of them prompting a smile from public defender Mark T. Wilson.
Mr Wilson declined to comment afterward.
"We'll have a lot to say at sentencing," he said.
LaRose and co-defendant Jamie Paulin-Ramirez of Colorado are the rare U.S. women charged with terrorism. Paulin-Ramirez has pleaded not guilty since she was arrested in Ireland with other terror suspects.
The March 2010 indictment charged LaRose with conspiring with jihadist fighters and pledging to commit murder in the name of a Muslim holy war, or jihad. The indictment was announced hours after authorities arrested seven suspected terrorists in Ireland allegedly linked to LaRose.
In emails recovered by the FBI over 15 months, LaRose agreed to marry an online contact from South Asia so he could move to Europe. She also agreed to become a martyr, the indictment said.
The man she had agreed to marry told her in a March 2009 email to go to Sweden to find the artist, Lars Vilks, who had depicted the Prophet Muhammad with the body of a dog, the indictment said.
Mr Vilks has questioned the sophistication of the plotters but said he is glad LaRose never got to him.
LaRose pleaded guilty on Tuesday to four counts: conspiracy to support terrorists, conspiracy to kill in a foreign country, lying to investigators and attempted identity theft.
