Rho guanosine triphosphatases (GTPases) regulate multiple aspects of dendritic cell (DC) function, but what regulates the expression of Rho GTPases in DCs is unknown. Here, we show that the extracellular matrix protein mindin regulates the expression of Rho GTPases in DCs. Mindin^−/− mice displayed defective CD4⁺ T‐cell priming and impaired humoral immune responses to T‐dependent antigens. Mindin^−/− DCs had reduced expression of Rac1/2 and impaired priming capacity owing to inefficient engagement with T lymphocytes. Ectopic Rac1 expression restored the priming capability of Mindin^−/− DCs. Furthermore, we show that DC adhesion to mindin matrix was blocked by antibodies to α₄, α₅ and β₁ integrins. DCs lacking β₁ integrin had reduced adhesion to mindin matrix, decreased expression of Rac1/2 and impaired priming capacity. These results suggest that mindin–integrin interactions play a key role in regulating Rho GTPase expression in DCs and DC priming of T lymphocytes.