A hallmark feature of amyotrophic lateral sclerosis (ALS) is that symptoms appear to spread along neuroanatomical pathways to engulf the motor nervous system, suggesting a propagative toxic entity could be involved in disease pathogenesis. Evidence for such a propagative entity emerged recently in studies using mice that express G85R-SOD1 mutant protein fused to YFP (G85R-SOD1:YFP). Heterozygous G85R-SOD1:YFP transgenic mice do not develop ALS symptoms out to 20 months of age. However, when newborns are injected with spinal homogenates from paralyzed mutant SOD1 mice, the G85R-SOD1:YFP mice develop paralysis as early as 6 months of age. We now demonstrate that injecting spinal homogenates from paralyzed mutant SOD1 mice into the sciatic nerves of adult G85R-SOD1:YFP mice produces a spreading motor neuron disease within 3.0 ± 0.2 months of injection. The formation of G85R-SOD1:YFP inclusion pathology spreads slowly in this model system; first appearing in the ipsilateral DRG, then lumbar spinal cord, before spreading rostrally up to the cervical cord by the time mice develop paralysis. Reactive astrogliosis mirrors the spread of inclusion pathology and motor neuron loss is most severe in lumbar cord. G85R-SOD1:YFP inclusion pathology quickly spreads to discrete neurons in the brainstem and midbrain that are synaptically connected to spinal neurons, suggesting a trans-synaptic propagation of misfolded protein. Taken together, the data presented here describe the first animal model that recapitulates the spreading phenotype observed in patients with ALS, and implicates the propagation of misfolded protein as a potential mechanism for the spreading of motor neuron disease.