Labialization is a secondary articulatory feature of sounds in some languages. Labialized sounds involve the lips while the remainder of the oral cavity produces another sound. The term is normally restricted to consonants. When vowels involve the lips, they are called rounded.

The most common labialized consonants are labialized velars. Most other labialized sounds also have simultaneous velarization, and the process may then be more precisely called labio-velarization.

Labialization may also refer to a type of assimilation process.

Labialization is the most widespread secondary articulation in the world's languages. It is phonemically contrastive in Northwest Caucasian (e.g. Adyghe), Athabaskan, and Salishan language families, among others. This contrast is reconstructed also for Proto-Indo-European, the common ancestor of the Indo-European languages.

American English has three degrees of labialization: tight rounded (/w/), slight rounded (/ʃ/, /ʒ/, /tʃ/, /dʒ/, initial /r/), and unrounded, which in vowels is sometimes called 'spread'. These secondary articulations are not universal. For example, French shares the English slight rounding of /ʃ/, /ʒ/ while Russian does not have slight rounding in its postalveolar fricatives (/ʂ ʐ ɕ ʑ/).