Proto-Slavic (abbreviated PSl., PS.; also called Common Slavic or Common Slavonic) is the unattested, reconstructed proto-language of all Slavic languages.

The Proto-Slavic language, the hypothetical ancestor of the modern-day Slavic languages, developed from the ancestral Proto-Balto-Slavic language, which is the parent language of the Balto-Slavic languages (both the Slavic and Baltic languages, e.g. Latvian and Lithuanian).