BACKGROUND Primary dysmenorrhea is a common and often debilitating condition affecting 40-90% of menstruating women. This condition reduces functionality, quality of life, and social activities. Transcranial direct current stimulation (tDCS) has been used in many chronic pain syndromes, with evidence of improved pain, functionality, and mood in women with primary dysmenorrhea. The objective of this study was to determine whether tDCS could offer clinical benefits on pain, anxiety, affectivity, and functionality in women with primary dysmenorrhea. METHODS This parallel, sham, randomized, double-blind trial was conducted with 26 women randomized into sham tDCS and active tDCS. Anodal tDCS was applied for 5 consecutive days over F3 corresponding to the left dorsolateral prefrontal cortex (DLPFC) and the cathode electrode over Fp2 for 20 min with an intensity of 2 mA. A numeric rating scale (NRS) was used to assess pain, anxiety, positive and negative affect, and submaximal aerobic performance during two consecutive menstrual cycles. RESULTS No significant interaction was found between intervention and time on the NRS [F(2,44) = 1.358, p = 0.26], and a significant main effect of time [F(2,44) = 4.446, p = 0.01] was found. The active group showed a significant reduction in anxiety (p = 0.03) with a mean difference of 5.12 (95% CI 0.79 to 11.05). No significant differences in positive and negative affect were found (p = 0.95 and p = 0.15, respectively). Submaximal aerobic performance was significantly greater in the active group [F(2,21) = 5.591, p = 0.02], with a mean difference of 70.87 (95% CI 8.53 to 133.21). CONCLUSIONS Anodal tDCS over the DLPFC seems to be an effective therapeutic approach for improving anxiety and functionality in women with primary dysmenorrhea.