Palatalization (/ˌpælətəlaɪˈzeɪʃən/ PAL-ə-təl-eye-ZAY-shən) is a historical-linguistic sound change that results in a palatalized articulation of a consonant or, in certain cases, a front vowel. Palatalization involves change in the place or manner of articulation of consonants, or the fronting or raising of vowels. In some cases, palatalization involves assimilation or lenition.

Palatalization is sometimes an example of assimilation. In some cases, it is triggered by a palatal or palatalized consonant or front vowel, but in other cases, it is not conditioned in any way.

Palatalization changes place of articulation or manner of articulation of consonants. It may add palatal secondary articulation or change primary articulation from velar to palatal or alveolar, alveolar to postalveolar.

It may also cause a consonant to change its manner of articulation from stop to affricate or fricative. The change in the manner of articulation is a form of lenition. However, the lenition is frequently accompanied by a change in place of articulation.

Palatalization of velar consonants commonly causes them to front, and apical and coronal consonants are usually raised. In the process, stop consonants are often spirantised except for palatalized labials.^{[citation needed]}

Palatalization, as a sound change, is usually triggered only by mid and close (high) front vowels and the semivowel [j]. The sound that results from palatalization may vary from language to language. For example, palatalization of [t] may produce [tʲ], [tʃ], [tɕ], [tsʲ], [ts], etc. A change from [t] to [tʃ] may pass through [tʲ] as an intermediate state, but there is no requirement for that to happen.

In the Nupe language, /s/ and /z/ are palatalized both before front vowels and /j/, while velars are only palatalized before front vowels. In Ciluba, /j/ palatalizes only a preceding /t/, /s/, /l/ or /n/. In some variants of Ojibwe, velars are palatalized before /j/, but apicals are not. In Indo-Aryan languages, dentals and /r/ are palatalized when occurring in clusters before /j/, but velars are not.

Palatalization sometimes refers to vowel shifts, the fronting of a back vowel or raising of a front vowel. The shifts are sometimes triggered by a nearby palatal or palatalized consonant or by a high front vowel. The Germanic umlaut is a famous example.