The wily creature evaded capture by police and caused havoc as it padded along the corridors over four stories of Parliament's Portcullis House building on Thursday night.
Julia Lopez, a lawmaker from the governing Conservative party, tweeted: "Team Lopez: just leaving Westminster office for the night & see a fox climbing escalator into Portcullis House!
"We have seen some strange things in Parliament since 2017, but this tops it!"
The fox had no qualms about showing lawmakers what it thought of them.
Kerry McCarthy, a Member of Parliament (MP) from the opposition Labour party, revealed that the interloper appeared to have defecated outside her office on the second floor.
"So apparently there was a fox running around in Portcullis House atrium about half an hour ago, and now I've just found this outside my office (2 floors up!) #Foxontherun," she tweeted.
Kate Ferguson, Westminster correspondent for The Sun newspaper, live tweeted the mayhem, reporting that police were in pursuit and trying to "shoo" the fox out of the building with "big gloves" as it "scarpered up to the third floor."
She added: "Fox News - the fox is now on the fourth floor...cops still on the case. But how do you catch a fox in a giant palace...?"
Social media users found much amusement in the spectacle of a different kind of pest on the loose at the heart of British politics.
Jo Johnson, who works in the Treasury, tweeted: "Some light relief from viruses, the Budget, US primaries etc ... as if there weren't enough fauna in Whitehall buildings already, apparently a fox got into Portcullis House."