In phonology, vowel harmony is an assimilatory process in which the vowels of a given domain – typically a phonological word – have to be members of the same natural class (thus "in harmony"). Vowel harmony is typically long distance, meaning that the affected vowels do not need to be immediately adjacent, and there can be intervening segments between the affected vowels. Generally one vowel will trigger a shift in other vowels, either progressively or regressively, within the domain, such that the affected vowels match the relevant feature of the trigger vowel. Common phonological features that define the natural classes of vowels involved in vowel harmony include vowel backness, vowel height, nasalization, roundedness, and advanced and retracted tongue root.

Vowel harmony is found in many agglutinative languages. The given domain of vowel harmony taking effect often spans across morpheme boundaries, and suffixes and prefixes will usually follow vowel harmony rules.