This study investigated whether there are sex differences in the spinal excitability of the human mirror-neuron system. We measured the modulation of spinal excitability, elicited by Hoffmann reflex in the left plantar flexor muscle (soleus), when women and men participants observed videos of bipedal heel-stepping (plantar dorsiflexion), standing still, and bipedal toe-stepping (plantar flexion). Men and women were similarly divided in their sex judgments of the observed legs. Our results indicate that whereas both men and women mimicked spinal excitability to the observed bipedal step, only women participants produced stronger modulation of spinal excitability. These findings demonstrate that the human mirror-neuron system exhibits sex differences in spinal excitability.