A notable sub-genre of fictional languages are alien languages, the ones that are used or might be used by putative extra-terrestrial lifeforms. Alien languages are subject of both science fiction and scientific research.

Perhaps the most fully developed fictional alien language is the Klingon language of the Star Trek universe - a fully developed constructed language.

The problem of alien language has confronted generations of science fiction writers; some have created fictional languages for their characters to use, while others have circumvented the problem through translation devices or other fantastic technology.

Although this field remains largely confined to science fiction, the possibility of intelligent extra-terrestrial life makes the question of alien language a credible topic for scientific and philosophical speculation. While many cases an alien language is but an element of fictional reality, in a number of science fiction works the core of the plot are linguistic and psychological problems of communication between various alien races.