Phonaesthetics (also spelled phonesthetics in North America) is the study of beauty and pleasantness associated with the sounds of certain words or parts of words. The term was first used in this sense, perhaps by J. R. R. Tolkien, during the mid-20th century and derives from Ancient Greek φωνή (phōnḗ) 'voice, sound', and αἰσθητική (aisthētikḗ) 'aesthetics'. Speech sounds have many aesthetic qualities, some of which are subjectively regarded as euphonious (pleasing) or cacophonous (displeasing). Phonaesthetics remains a budding and often subjective field of study, with no scientifically or otherwise formally established definition; today, it mostly exists as a marginal branch of psychology, phonetics, or poetics.

More broadly, the British linguist David Crystal has regarded phonaesthetics as the study of "phonaesthesia" (i.e., sound symbolism and phonesthemes): that not just words but even certain sound combinations carry meaning. For example, he shows that English speakers tend to associate unpleasantness with the sound sl- in such words as sleazy, slime, slug, and slush, or they associate repetition lacking any particular shape with -tter in such words as chatter, glitter, flutter, and shatter.