Jaelyn Delshaun Young, 20, was arrested at a Mississippi airport in August 2015 while attempting to board a flight to Turkey with her husband, Muhammad Oda Dakhlalla, 23.
Young acknowledged her role as the "planner of the expedition" in an incriminating farewell letter, according to court documents filed by U.S. prosecutors.
Convictions for Islamic State-related activity by Americans have become more frequent in recent months as more than 80 such cases brought by U.S. prosecutors since 2013 work their way through federal courts.
Young's Twitter posts about her desire to join the militant group caught the attention of the FBI in May 2015, and an agent posing as an Islamic State recruiter began corresponding with her and Dakhlalla.
Both Dakhlalla and Young, of Starkville, Mississippi, are U.S. citizens. Young converted to Islam in March 2015, the court documents said.
The couple entered their guilty pleas in the U.S. District Court for the Northern District of Mississippi, in Greenville.