Language is a strong and robust determinant of international trade patterns: Countries sharing a common language trade significantly more with each other than countries using different languages, holding other factors constant. In this article we present the first analysis of the effect of language on trade in an intra‐national context. Analyzing unique data for a single‐language country, Germany, we find that similarities in the local dialect have a significantly positive impact on regional trade. We interpret this finding as evidence for the trade‐promoting effect of culture, because linguistic similarities likely reflect cultural ties across regions, rather than lower costs of communication or similar institutions.