In phonology, epenthesis (/ɪˈpɛnθəsɪs, ɛ-/; Greek ἐπένθεσις) means the addition of one or more sounds to a word, especially in the first syllable (prothesis), the last syllable (paragoge), or between two syllabic sounds in a word. The opposite process in which one or more sounds are removed is referred to as syncope or elision.

The word epenthesis comes from epi- 'in addition to' and en- 'in' and thesis 'putting'. Epenthesis may be divided into two types: excrescence for the addition of a consonant, and for the addition of a vowel, svarabhakti (in Sanskrit) or alternatively anaptyxis (/ˌænəpˈtɪksɪs/).

Epenthesis arises for a variety of reasons. The phonotactics of a given language may discourage vowels in hiatus or consonant clusters, and a consonant or vowel may be added to help pronunciation. Epenthesis may be represented in writing, or it may be a feature only of the spoken language.

A consonant may be added to separate vowels in hiatus, as is the case with linking and intrusive R in English.

A consonant may be placed between consonants in a consonant cluster where the place of articulation is different (such as if one consonant is labial and the other is alveolar).

A vowel may be placed between consonants to separate them.

While epenthesis most often occurs between two vowels or two consonants, it can also occur between a vowel and a consonant or at the ends of words. For example, the Japanese prefix ma- (真〜（ま〜）; 'pure …, complete …') transforms regularly to ma'- (真っ〜（まっ〜）; (gemination of following consonant)) when it is followed by a consonant, as in masshiro (真っ白（まっしろ）; 'pure white'). The English suffix -t, often found in the form -st, as in amongst (from among + -st), is an example of terminal excrescence.

Excrescence is the epenthesis of a consonant.