双元音(diphthong)是一种复元音(compound vowel)，又称双元音或二合复元音，在语音学中，指联合的两个元音，它们作为一个整体出现。之间有平滑的过渡，也就是说，双元音中的发音牵涉到两种不同的舌位，并且从其中一种舌位滑动到另一舌位。在国际音标 (IPA) 中，单元音用一个字母表示，如[ʌ]。双元音用两个字母表示，如[aɪ]，响度较低的字母可以加上  ̯ 符号以表示它是双元音的一部份，例如 [aɪ̯]。

A diphthong (/ˈdɪfθɒŋ/ DIF-thong or /ˈdɪpθɒŋ/ DIP-thong; from Greek: δίφθογγος, diphthongos, literally "two sounds" or "two tones"), also known as a gliding vowel, is a combination of two adjacent vowel sounds within the same syllable. Technically, a diphthong is a vowel with two different targets: that is, the tongue (and/or other parts of the speech apparatus) moves during the pronunciation of the vowel. In most dialects of English, the phrase no highway cowboys /ˌnoʊ ˈhaɪweɪ ˈkaʊbɔɪz/ has five distinct diphthongs, one in every syllable.