A 68-year-old man with a diagnosis of transformation to undifferentiated carcinoma of the left thyroid who was being treated with Lenvatinib presented with swelling and pain around the left clavicle, and tramadol was started. Two days later, he developed diarrhea, sweating, disorientation, and myoclonus, leading to a diagnosis of serotonin syndrome. He also exhibited dyskinesia including involuntary movements of the arms and legs and squirming movements of the trunk. Tramadol was thus discontinued. His myoclonus and dyskinesia resolved within half a day and had disappeared the day after tramadol discontinuation. These symptoms were attributed to tramadol because there was a reasonable temporal relationship between drug administration and the adverse event. Tramadol inhibits serotonin reuptake, and thus has the potential to cause serotonin syndrome. However, there have been relatively few reports describing the occurrence of this syndrome, and there have been none showing concomitant dyskinesia. Clinicians should be aware that tramadol can cause serotonin syndrome accompanied by dyskinesia.