In phonology, epenthesis ( /əˈpɛnθəsɪs/; Ancient Greek: ἐπένθεσις, epenthesis from epi "on" + en "in" + thesis "putting") is the addition of one or more sounds to a word, especially to the interior of a word. Epenthesis may be divided into two types: excrescence, for the addition of a consonant, and anaptyxis (/ˌænæpˈtɪksɨs/) for the addition of a vowel.

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 make pronunciation easier.

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

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

A vowel may be placed between consonants to separate them.

In French, /t/ is inserted in inverted interrogative phrases between a verb ending in a vowel and a pronoun beginning with a vowel, such as il a ('he has') > a-t-il ('has he?'). Here there is no epenthesis from a historical perspective, since the a-t is derived from Latin habet (he has), and the t is therefore the original third person verb inflection. However it is correct to call this epenthesis when viewed synchronically, since the modern basic form of the verb is a, and the psycholinguistic process is therefore the addition of t to the base form.