Metal oxide nanostructures are widely applied as electrode surface modifiers in modern electroanalytical chemistry. Among them, manganese dioxide nanorods are of interest and has been applied in colorants electroanalysis. An electrode modified with manganese dioxide nanorods dispersed in cationic surfactant hexadecylpyridinium bromide has been developed for the quantification of rosmarinic acid. The application of hexadecylpyridinium bromide as dispersive agent provides stabilization of nanomaterial suspension in water medium. The electrode developed gives an improved response to rosmarinic acid, i.e. cathodic to anodic peak potential separation of 60 mV and 1.7-fold increased redox currents have been observed. Quasi-reversible electrooxidation controlled by surface processes has been confirmed. Differential pulse voltammetry in Britton-Robinson buffer pH 5.0 has been applied for the quantification of rosmarinic acid. Linear dynamic ranges of 0.025-1.0 and 1.0-10 μM with a detection limit of 9.7 nM have been achieved that are significantly improved compared to other electrochemical methods using modified electrodes. The selectivity of the electrode response to rosmarinic acid has been shown in the presence of a 1000-fold excess of inorganic ions, 100-fold excesses of saccharides, and 10-fold excesses of ascorbic and p-coumaric acids, eugenol, carvacrol, and thymol. Other phenolic acids (gallic, ferulic, caffeic) and flavonoids (quercetin, rutin) give an interference effect. The practical application of the electrode developed has been demonstrated on rosemary spices.