Fox official Paul Schur said on Sunday chief political correspondent Carl Cameron had published a fake story with several made-up quotes attributed to Kerry.
The article lied about a post-debate rally at which the Massachusetts senator was purported to gush over his "metrosexual" appearance.
"Didn't my nails and cuticles look great? What a good debate!" the article by Cameron read, purportedly quoting Kerry after the event.
"Women should like me! I do manicures," the story also quotes Kerry as telling the crowd. The article also has the Democratic candidate contrasting himself to US President George Bush.
"I'm metrosexual - he's a cowboy," Cameron quoted Kerry as saying.
Officials for Fox, which has been criticised for being partial to the Republican party, declined to explain how the spoof article ended up on the network's website.
A statement by Fox on the website on Sunday apologised for the article, saying it was a joke.
"Foxnews.com erred ... on Friday, posting an item purporting to contain quotes attributable to Kerry," it said.
"The item was based on a reporter's partial script that had been written in jest and should not have been posted or broadcast. Foxnews.com also regrets that error, which occurred because of fatigue and bad judgment, not malice."