The noun referring to one who takes an assumed identity in order to deceive is variously spelled imposter and impostor. Impostor has the edge, and it is the form recommended by most English reference sources, but imposter is not wrong. Not only is it nearly as common as impostor, but it is also nearly as old. Impostor came to English from the French imposteur in the late 16th century,1 and imposter first appeared almost immediately thereafter.2 And though the -or spelling has always been more common, imposter has always been present to some degree.
In fact, imposter is more common than impostor in some areas of English. In a search covering a few dozen of the most popular blogs in the English-speaking world, for instance, the ratio of imposter to impostor is about 6:5. Imposter is also a little more common than impostor in 21st-century Australian and New Zealand news publications that make content available online. The two forms are neck and neck in British and American news publications from this century. Meanwhile, in a Google Books search—which covers millions books, journals, and magazines—limited to 2000 to the present, impostor remains about three times more common than imposter.
If you came here looking for a simple answer, go with impostor, but only because that’s the spelling you’ll find recommended in dictionaries, usage guides, and such, which makes it the safer choice.
Instances of both spellings are easy to find in all sorts of writing from around the English-speaking world—for example:
Far from being an expression of humour, confidence or even friendliness, the smile seemed false, an imposter at a funeral. [The Australian]
I felt something of an impostor being treated as a “victim.” [Independent]
False acceptance probability is the area to the right of the threshold under the imposter distribution. [Encyclopedia of Biometrics, Stan Z. Li (2009)]
She felt the prince’s gaze on her back, burning through her clothing, branding her, seeming to call her out for the impostor she was. [Wicked in Your Arms, Sophie Jordan (2011)]
At any rate, at least we have a face to put to the name, even if it’s an imposter. [Mashable (2012)]
For instance, we would expect a subject with Capgras delusion, convinced that his wife has been replaced by an impostor, to look for his real wife, and show hostile behaviour to the alleged impostor. [Neuroethics (2012)]