Amyotrophic lateral sclerosis (ALS) is a deadly neurodegenerative disease characterized by loss of motor neurons, paralysis and eventual death. The mechanism of ALS is still incompletely understood, and the disease is to date without an effective remedy. Protein arginine deiminase 2 (PAD2) converts peptidyl-Arginine into peptidyl-Citrulline, a post-translational modification referred to as citrullination. Aberrant expression of PAD2 and protein citrullination are increased in several neurodegenerative conditions. Whether this increase is involved in ALS is unknown. In this study, we investigated PAD2 and protein citrullination in two genetic mouse models of ALS expressing human mutant SOD1^G93A and PFN1^C71G, respectively, and in post-mortem human ALS spinal cord. We show that the expression of PAD2 mRNA and protein expressions are increased progressively along with the ALS progression. Additionally, protein citrullination is increase following the same trend. These changes occur in areas with the most severe motor neuron degeneration including the spinal cord, and brainstem. We show that the increase in PAD2 and citrullinated proteins occur specifically in astrocytes coinciding with the development of reactive astrogliosis. Finally, we show that citrullinated proteins form non-astrocyte aggregate patterns; and are dominantly expressed in insoluble protein fractions. These results demonstrate that PAD2 dysregulation and increased protein citrullination are key characteristics of reactive astrogliosis, and possibly drive some type of protein aggregation in the pathogenesis of ALS. Because protein citrullination alters protein functions, our results suggest that PAD2 and protein citrullination play a role in astrogliosis and astrocytic toxicity in ALS and other neurodegenerative conditions.