Up to 20% of patients with behavioural variants of frontotemporal dementia (FTD) also have motor neuron disease (MND); conversely, this comorbidity is rare in patients with language variants of FTD. A few patients have been reported with semantic dementia (SD) combined with MND. However, these patients demonstrated the clinical features of MND in the advanced stage. We report a patient with SD who also demonstrated MND symptoms in an earlier stage of the disease. A 61-year-old man visited our memory disorder clinic as a result of language disturbance and dysarthria of 8 months duration and facial recognition impairment of 3 months duration. Neuropsychological tests revealed anomic aphasia, prosopagnosia, and decreased semantic fluency. A brain MRI revealed significant atrophies localized in both anterior temporal lobes with a greater prominence on the right side. Clinical examination and electrophysiological studies confirmed a diagnosis of MND 17 months after the onset of the disease.