Kittleson was abducted by the Iran-backed Iraqi militia Kataib Hezbollah from a street corner in Baghdad on March 31.
Rubio confirmed her release in a statement posted on social media platform X.
"We are relieved that this American is now freed and are working to support her safe departure from Iraq," he wrote.
Rubio thanked Iraqi authorities, as well as the FBI, Defence Department and other US agencies for their work toward securing Kittleson's release.
Kataib Hezbollah had said in a statement earlier it had decided to free Kittleson on condition she must leave the country immediately upon her release.
Two officials within the militia, who spoke on condition of anonymity, told The Associated Press that in exchange for freeing Kittleson, several members of the group who had previously been detained by Iraqi authorities would be released.
In its statement, Kataib Hezbollah said its decision came "in appreciation of the patriotic stances of the outgoing" Iraqi Prime Minister Mohammed Shia al-Sudani, without giving more details.
It added that "this initiative will not be repeated in the future".
Kittleson, 49, a freelance journalist, had lived abroad for years before the kidnapping, using Rome as her base for a time and building a respected journalism career across the Middle East, particularly in Iraq and Syria.
Like many freelancers, she often worked on a shoestring budget and without the protections afforded by large news organisations to staff.
She had entered Iraq again shortly before her abduction. US officials have said they warned her multiple times of threats against her, but she did not want to leave.
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