In linguistics, vowel length is the perceived length of a vowel sound: the corresponding physical measurement is duration. In some languages vowel length is an important phonemic factor, meaning vowel length can change the meaning of the word, for example in Arabic, Estonian, Finnish, Fijian, Japanese, Kannada, Kyrgyz, Latin, Malayalam, Old English, Scottish Gaelic, and Vietnamese. While vowel length alone does not change word meaning in most dialects of English, it is said to do so in a few dialects, such as Australian English, Lunenburg English, New Zealand English, and South African English. It also plays a lesser phonetic role in Cantonese, unlike in other varieties of Chinese.

Many languages do not distinguish vowel length phonemically, meaning that vowel length does not change meaning, and the length of a vowel is conditioned by other factors such as the phonetic characteristics of the sounds around it, for instance whether the vowel is followed by a voiced or a voiceless consonant. Languages that do distinguish vowel length phonemically usually only distinguish between short vowels and long vowels. Very few languages distinguish three phonemic vowel lengths, such as Estonian, Luiseño, and Mixe. However, some languages with two vowel lengths also have words in which long vowels appear adjacent to other short or long vowels of the same type: Japanese hōō, "phoenix", or Ancient Greek ἀάατος [a.áː.a.tos], "inviolable". Some languages that do not ordinarily have phonemic vowel length but permit vowel hiatus may similarly exhibit sequences of identical vowel phonemes that yield phonetically long vowels, such as Georgian გააადვილებ [ɡa.a.ad.vil.eb], "you will facilitate it".