Shannon Conley, a 19-year-old Muslim convert, was arrested in April while trying to board a flight to Turkey en route to Syria to marry an IS fighter.
Prosecutors offered a reduced term if she helped share information about other Americans looking to join IS.
Conley, who now calls herself Halima, said she deeply regrets her actions.
Handing down the verdict at a court in Denver, Judge Raymond Moore said the sentence was meant to deter others who wanted to join Islamic militants.
The judge also expressed doubt about Conley?s claim that she had disavowed jihad.
?Defiance has been a part of her fabric for a long time,? he said, adding that Conley needed mental help.
Conley appeared in the courtroom wearing a headscarf with her prison uniform. She had earlier pleaded guilty to one count of conspiracy to provide material support to a foreign terrorist organisation.
She had faced up to five years in prison.
Conley, who is a nurse?s assistant, told her parents she planned to marry Yousr Mouelhi, who she met online and believed to be a Tunisian IS fighter.
The FBI became interested in Conley after she alarmed employees of a church in Denver by taking notes on the layout of the building.
Over the course of eight months, FBI agents repeatedly tried to discourage her from travelling abroad, suggesting she explore humanitarian work instead.
But her father, who had refused to let her marry her Tunisian suitor, discovered a one-way ticket to Turkey with Conley?s name on it.