Fair Witness is a fictional profession invented for the novel "Stranger in a Strange Land" by R.A. Heinlein. A Fair Witness is an individual trained to observe events and report exactly what he or she sees and hears, making no extrapolations or assumptions. An eidetic memory is a prerequisite for the job, although this may be attainable with suitable training.

In Heinlein's society, a Fair Witness is a highly reputable source of information. By custom, a Fair Witness acting professionally, generally wearing distinctive white robes, is never addressed directly, and all present are supposed to avoid acknowledging the presence of the Witness in any way.

The character Jubal Harshaw employs a Fair Witness, Anne, as one of his secretaries. Unlike the other secretaries, she does not use dictation equipment when Jubal speaks. She can even keep track of several works at once, despite Harshaw's frequent switching among them.

Fair Witnesses are prohibited from drawing conclusions about what they observe. As a demonstration, Harshaw asks Anne to describe the color of a house in the distance. She responds, "It's white on this side". Harshaw explains that she would not assume knowledge of the color of the other sides of the house without being able to see them. Furthermore, after observing another side of the house would not then assume that any previously seen side was still the same color as last reported, even if only minutes before.

Ane Truthtellers are reported to have abilities the equivalent of the Fair Witness. This claim is being carefully probed by both science and law.