Soft science fiction, or soft SF is a category of science fiction that is based on and explores the "soft" sciences, and especially the social sciences (anthropology, sociology, psychology, political science, and so on), rather than engineering or the "hard" sciences (for example, physics, astronomy, or chemistry). Soft science fiction is often more concerned with character, and speculative societies rather than scientific or engineering speculations. It is the complement of hard science fiction. The term first appeared in the late 1970s and early '80s.

In The Encyclopedia of Science Fiction, Peter Nicholls writes that "soft SF" is a "not very precise item of SF terminology" and that the contrast between hard and soft is "sometimes illogical." In fact, the boundaries between "hard" and "soft" are neither definite nor universally agreed-upon, so there is no single standard of scientific "hardness" or "softness." Some readers might consider any deviation from the possible or probable (for example, including faster-than-light travel or paranormal powers) to be a mark of "softness." Others might see an emphasis on character or the social implications of technological change (however possible or probable) as a departure from the science-engineering-technology issues that in their view ought to be the focus of hard SF. Given this lack of objective and well-defined standards, "soft science fiction" does not indicate a genre or subgenre of SF but a tendency or quality—one pole of an axis that has "hard science fiction" at the other pole.

For example, a book like George Orwell's Nineteen Eighty-Four might be described as soft science fiction, since it is concerned primarily with how society and interpersonal relationships are altered by a political force which uses technology mercilessly; even though it is the source of many ideas and tropes commonly explored in subsequent science fiction, (even in hard science fiction), such as mind control and surveillance. And yet, its style is uncompromisingly realistic, and despite its then-future setting, very much more like a spy novel or political thriller in terms of its themes and treatment.

Another example of soft science fiction would be the film The Invention of Lying, written by Ricky Gervais which explores an alternate setting where lying is impossible. However, the setting appears to be relatively similar to our own world in terms of aesthetics and industrial progression.

By contrast, Karel Čapek's R.

U.

R., though the original source for the robot found with near-ubiquity in much subsequent Science Fiction, is un-scientific in its style and approach. Čapek's robots are manufactured like gingerbread men, implying that for all his concerns with free will and ideas of personal liberty, his play is essentially a kind of soft science fiction, or even a literary fantasy.