In phonetics, a triphthong (UK: /ˈtrɪfθɒŋ, ˈtrɪpθɒŋ/ TRIF-thong, TRIP-thong, US: /-θɔːŋ/ -⁠thawng) (from Greek τρίφθογγος triphthongos, lit. 'with three sounds' or 'with three tones') is a monosyllabic vowel combination involving a quick but smooth movement of the articulator from one vowel quality to another that passes over a third. While "pure" vowels, or monophthongs, are said to have one target articulator position, diphthongs have two and triphthongs three.

Triphthongs are not to be confused with disyllabic sequences of a diphthong followed by a monophthong, as in German Feuer [ˈfɔʏ.ɐ] 'fire', where the final vowel is longer than those found in triphthongs.

Triphthongs that feature close elements typically analyzed as /j/ and /w/ in phonology are not listed. For instance, the Polish word łój [wuj] 'tallow' is typically analyzed as /CVC/ - a sequence of a consonant followed by a vowel and another consonant. This is because the palatal approximant is resyllabified in some inflected forms, such as łojami [wɔˈjami] (instr. pl.), and also because /w/ occurs word-finally after a consonant just like /l/ does (compare przemysł [ˈpʂɛmɨsw] 'industry' with Przemyśl [ˈpʂɛmɨɕl] 'Przemyśl'), which means that both of them behave more like consonants than vowels.

On the other hand, [ɪ̯, i̯, ʊ̯, u̯] are not treated as phonetic consonants when they arise from vocalization of /l/, /v/ or /ɡ/ as they do not share almost all of their features with those three.

Bernese German has the following triphthongs:

They have arisen due to the vocalization of /l/ in the syllable coda; compare the last two with Standard German Gefühl [ɡəˈfyːl] and Schule [ˈʃuːlə], the last one with a schwa not present in the Bernese word.

Danish has the following triphthongs:

In British Received Pronunciation, and most other non-rhotic (r-dropping) varieties of English, monosyllabic triphthongs with r are optionally distinguished from sequences with disyllabic realizations: