Soft science fiction or soft SF is science fiction whose plots and themes tend to focus on human feelings, while de-emphasizing the details of technological hardware and physical laws. In addition, 'science' in soft science fiction often falls into the realm of things which current scientists consider impossible or at least highly unlikely. It is called soft science fiction by analogy to hard science fiction and because soft science fiction is often based around the 'softer' sciences (philosophy, psychology, politics and sociology). "Soft SF" is also used as a synonym for the "New Wave", a movement which emerged in the 1970s.

Soft SF is much less a defined subgenre than its counterpart, Hard science fiction. The term is sometimes used in a pejorative fashion when it is implied a given science fiction story is not rigorous enough in its application of science or is not "proper" science fiction. Contrariwise, patrons of Soft SF may claim that their preferred works have stronger portrayals of societies, more deft characterization and better-developed plots.

One could make an argument, for example, classifying Isaac Asimov's Foundation series as part of the "soft" subgenre, since the series focuses on the vast sociological movements of the dying Galactic Empire. Asimov, so this argument goes, places little emphasis on the specifics of his fictional technologies. It is enough that the Foundation is technologically superior to the "barbarian" planets around it: the details of nuclear power plants don't matter, as long as the Foundation is the only one to possess them. On the other hand, one of the most frequent comments made about Asimov's work is that his stories lack description, and that there are few sharply memorable characters scattered throughout the whole Foundation epic; this would seem to go against the grain of the argument that Soft SF necessarily has deeper characterization. Furthermore, Asimov treats his "soft sciences" in a remarkably "hard" way: his fictional science of psychohistory is a mathematical way of encapsulating the "human texture" of his sociological story.

One classic example of a Soft SF writer is Ray Bradbury. (Asimov himself used Bradbury to typify the "emotional" style of writing he seldom employed; for examples of this usage, see the correspondence collection Yours, Isaac Asimov.) In Bradbury's short stories, such as those collected in R is for Rocket and The Martian Chronicles he takes common themes in Hard SF, like rocket travel or Mars colonies, but focuses on the feelings and human responses those themes evoke. In 1955, the Argentine writer Jorge Luis Borges pinpointed this function of Bradbury's Chronicles, observing that

In this outwardly fantastic book, Bradbury has set out the long empty Sundays, the American tedium, and his own solitude, as Sinclair Lewis did in Main Street.

Perhaps "The Third Expedition" is the most alarming story in this volume. Its horror (I suspect) is metaphysical; the uncertain identity of Captain John Black's guests disturbingly insinuates that we too do not know who we are, nor what we look like in the eyes of God. I would also like to note the episode entitled "The Martian," which includes a moving variation on the myth of Proteus.

Frank Herbert's Dune arguably falls into this category, though of course its fans are quick to deny any pejorative implications of the usage.

Compare: hard science fiction, space opera, comic science fiction, social fiction