In phonetics, vowel roundedness refers to the amount of rounding in the lips during the articulation of a vowel. That is, it is vocalic labialization. When pronouncing a rounded vowel, the lips form a circular opening, while unrounded vowels (also called spread vowels) are pronounced with the lips relaxed. In most languages, front vowels tend to be unrounded, while back vowels tend to be rounded. But some languages, such as French and German, distinguish rounded and unrounded front vowels of the same height, while, for example, Vietnamese distinguishes rounded and unrounded back vowels of the same height.

In the International Phonetic Alphabet vowel chart, rounded vowels are the ones that occur on the right in each pair of vowels. There are also diacritics, respectively ɔ̹ ɔ̜, to indicate greater or lesser degrees of rounding. The 'more' and 'less rounded' diacritics are sometimes also used with consonants to indicate degrees of labialization. (See relative articulation.)