An analytical method based on an optimized solid-phase extraction procedure and followed by high-performance liquid chromatography (HPLC) separation with diode array detection was developed and validated for the determination of rosmarinic acid in leaves of Mentha piperita L. The chromatographic determination of rosmarinic acid was carried out with a Phenomenex Synergi 4u Fusion-RP C18 (250 x 4.6 mm i.d., 4 μm particle size), as stationary phase, with a flow rate of 1 mL/min and detection at a wavelength of 330 nm. The proposed method was validated by ICH Harmonised Tripartite Guidelines “Validation of analytical procedures: Text and Methodology Q2(R1)”. In this study, an excellent linearity was obtained with r higher than 0.99. With other validation data, including precision, specificity, accuracy and robustness, this method demonstrated good reliability and sensitivity, and can be conveniently used for the quantification of rosmarinic acid in leaves of Mentha piperita L. In summary, the method above can be considered specific, exact, precise, linear, robust and easy to perform. Further this method can be applied to a standardization of multicomponent herbal remedies, that incorporate leaves of Mentha piperita L.