The satirical story, with the headline "The Onion: Frustrated Obama Sends Nation Rambling 75,000-Word E-Mail," appeared in the Fox Nation section of the site Friday.
Beside it was the more serious headline, "China Issues Warning Ahead of U.S.-South Korea Drills."
The original Obama headline didn't mention The Onion, and the only clue for readers was a link to the satirical site after the first two paragraphs of the story.
The Obama item was marked as a "culture" story, as are some of the opinion pieces intermixed with news stories on the site. It was finally taken down early Friday evening.
But the fake news of the president's novel-length missive remained on the site for several hours, even after Mediaite pointed it out -- and after Fox News updated its site with breaking news that the president needed 12 stitches for a basketball injury.
While the mention of The Onion in the headline would tip off fans of the satire site that the story was a joke, the article confused at least some Fox News readers.
"I can't seem to confirm this story anywhere except Onion. If it is false, somebody needs to go to jail for it," one wrote. "If it is true, this dude is crazier than an outhouse rat and needs some serious professional help! Someone needs to get to the bottom of this story and produce some verifiable facts to decide which way it should go."
Wrote another: "It is really sad that in a time of crisis in this country with troops on foreign soil the economy lack of jobs and now N. Korea once again rattling their saber we have a President sitting in the White House that by what I read in the email is utterly losing it. We can not afford to have him in the White House at such a critical time. (If this is real that is)."
Onion stories are mistakenly passed off as real news all the time, usually by those distant uncles you just ate turkey with. But it's unclear in this case whether the site mistook the story for real, or just failed to label it clearly for readers unfamiliar with The Onion. A Fox News rep did not immediately respond to a request for comment Friday.
Also, in case we've reached the point where it's now considered okay to post satire alongside real news and expect readers to sort out which is which, a quick clarification: This story is totally real.