Voiceless alveolar fricatives are a type of fricative consonant pronounced with the tip or blade of the tongue against the alveolar ridge (gum line) just behind the teeth. This refers to a class of sounds, not a single sound. There are at least six types with significant perceptual differences:

The first three types are sibilants, meaning that they are made by directing a stream of air with the tongue towards the teeth and have a piercing, perceptually prominent sound.

A voiceless alveolar sibilant is a common consonant sound in vocal languages. It is the sound in English words such as sea and pass, and is represented in the International Phonetic Alphabet with ⟨s⟩. It has a characteristic high-pitched, highly perceptible hissing sound. For this reason, it is often used to get someone's attention, using a call often written as sssst! or psssst!.

Voiceless alveolar sibilants [s] are one of the most common sounds cross-linguistically. If a language has fricatives, it will most likely have [s]. However, some languages have a related sibilant sound, such as [ʃ], but no [s]. In addition, sibilants are absent from most Australian Aboriginal languages, in which fricatives are rare; however, [s] does occur in Kalaw Lagaw Ya.

The term "voiceless alveolar sibilant" is potentially ambiguous in that it can refer to at least two different sounds. Various languages of northern Iberia (e.g., Astur-Leonese, Catalan, Basque, Galician, Portuguese and Spanish) have a so-called "voiceless apico-alveolar sibilant" that lacks the strong hissing of the [s] described in this article but has a duller, more "grave" sound quality somewhat reminiscent of a voiceless retroflex sibilant. Basque, Mirandese and some Portuguese dialects in northeast Portugal (as well as medieval Spanish and Portuguese in general) have both types of sounds in the same language.

There is no general agreement about what actual feature distinguishes these sounds. Spanish phoneticians normally describe the difference as apical (for the northern Iberian sound) vs. laminal (for the more common sound), but Ladefoged and Maddieson claim that English /s/ can be pronounced apically, which is evidently not the same as the apical sibilant of Iberian Spanish and Basque. Also, Adams asserts that many dialects of Modern Greek have a laminal sibilant with a sound quality similar to the "apico-alveolar" sibilant of northern Iberia.

Some authors have instead suggested that the difference lies in tongue shape. Adams describes the northern Iberian sibilant as "retracted". Ladefoged and Maddieson appear to characterize the more common hissing variant as grooved, but there is some doubt about whether all and only the "hissing" sounds actually have a "grooved" tongue shape.

Features of a voiceless alveolar sibilant: