•Serine 403-phosphorylated p62 were found in the postmortem tissues of ALS and AD.•p62 dephosphorylation and proteolysis may occur in postmortem human pathology.•p62 dephosphorylation occurred in the brains by postmortem delay in HD model mice. Protein inclusions in neurodegenerative diseases are associated with p62, which has an important role in autophagic clearance of polyubiquitinated proteins. Selective autophagy is regulated by S403-phosphorylation of p62, and S403-phosphorylated p62 (S403-phos-p62) accumulates in Atg5 conditional knockout (Atg5CKO) mice in which autophagosome formation is impaired. We performed immunohistochemical tests for the presence of S403-phos-p62 in postmortem brain of neurodegenerative disease cases, and found accumulations in amyotrophic lateral sclerosis and Alzheimer's disease tissues. In Atg5CKO and HD190QG (Huntington's disease model) mice, however, we found a postmortem decrease in S403-phos-p62 immunoreactivity, suggesting that post-mortem changes should be considered when interpreting human data.