In the present study, we explore sex differences of neuroplasticity in humans, as revealed by transcranial direct current stimulation, which induces motor cortical excitability changes both during and after stimulation. We retrospectively re-analyzed data collected from previous transcranial direct current stimulation studies. In women, the excitability-diminishing after-effects of cathodal transcranial direct current stimulation were relevantly prolonged compared with the male group. Similarly, during a short direct current stimulation that elicits no after-effects, the female group showed more inhibition. In contrast, no significant differences between male and female study participants were found for excitability-enhancing anodal transcranial direct current stimulation. These results suggest sex differences, possibly due to the effects of sex hormones, in the modulation of human cortical plasticity.