Hard fantasy is a sub-genre of fantasy literature that strives to present stories set in (and often centered on) a rational and knowable world. Hard fantasy is similar to hard science fiction, from which it draws its name, in that both aim to build their respective worlds in a rigorous and logical manner. The two diverge in that hard science fiction uses real scientific principles as its starting point, while hard fantasy postulates starting conditions that do not, and often cannot, exist according to currently scientific understanding.

Unlike its sister genre, the definition of hard fantasy is amorphous in practice. Some instances of the genre feature alternative geography and cultures without the presence of magic, dragons, and elves stereotypically found in many other fantasy settings. Other hard-fantasy settings may feature those elements but with a more detailed explanation for their existence.