Under terms of the agreement, Asher Abid Khan, 23, of Spring, Texas, must fully cooperate with any further federal investigation of his case. In return, prosecutors will drop five other counts of the indictment against him, including three conspiracy counts.
Khan could get up to 15 years in prison, but prosecutors agreed not to oppose leniency in sentencing if he's true to his agreement. If he had gone to trial and been convicted of all charges, Khan could have been sentenced to life in a federal prison.
Prosecutors initially alleged Khan used social media to join and support IS, but his attorney has said he used social media to persuade another Texas man not to join the IS.
The indictment alleged that Khan, a US citizen, travelled from Australia to Turkey in February 2014 as part of a plan to join the Islamic State in Syria. But once he got to Turkey, Khan had a change of heart and returned to his home in suburban Houston.
He was supposed to rendezvous in Turkey with Sixto Garcia, of McAllen, Texas, who authorities said went on to join the group and was killed in fighting later that year.
According to court documents, investigators allege that in one message on Facebook, Khan asked, "I'd like to join."
Khan's attorney, Thomas Berg, has contended that his client went online to try to persuade Garcia to not join the Islamic State.
The case was seen as a reflection of a growing use of social media by authorities to prosecute US citizens suspected of trying to aid terrorists.
"We aggressively investigate and prosecute persons who provide material support to terrorist organisations," said Abe Martinez, acting US attorney for the Southern District of Texas. Martinez said Khan provided IS "a battlefield soldier to further the terrorist organisation's violent agenda".