The Vasconic substrate hypothesis is a proposal that several Western European languages contain remnants of an old language family of Vasconic languages, of which Basque is the only surviving member. The proposal was made by the German linguist Theo Vennemann, but has been rejected by other linguists.

According to Vennemann, Vasconic languages were once widespread on the European continent before they were mostly replaced by Indo-European languages. Relics of these languages include toponyms across Central and Western Europe.

Theo Vennemann based his hypothesis on the works of Hans Krahe, who postulated an Old European substrate as the origin of the European hydronymy (Old European hydronymy). He classified the substratum language as Indo-European.

Vennemann rejected the classification. He gives the following reasons:

Vennemann developed his ideas in a series of papers which were collected in a book called Europa Vasconica–Europa Semitica.

He accepts Krahe's theory that there was a uniform Old European language, which is the origin of the Old European hydronymy, but proposes that it is of Vasconic origin. Vasconic is a language family proposed by Vennemann encompassing Basque (its only extant member), Aquitanian, Ligurian, and possibly Iberian and Proto-Sardinian.

Vennemann proposes that this uniform Vasconic substrate must come from a linguistically uniform population, which can only exist within a small area. He therefore proposes that during the last Ice Age, the Vasconic people lived in the modern region of Aquitania. As the glaciers retreated, the Vasconics began moving to the north and south, settling most of Western and Central Europe, spreading their language. They gave names to the rivers and places. This toponymy mostly persisted after the Vasconic language was replaced by Indo-European languages in most of their area, of which the present Basque area in northern Spain and southern France is postulated to be a relic.

In support of this argument, Vennemann cites, inter alia: