Increased oxidative stress (OS) and the disruption of the equilibrium between the production of reactive oxygen species and antioxidants are key molecular features of unhealthy aging. OS results in the formation of oxidative posttranslational modifications (PTMs), some of which involve cysteine (Cys) residues in aging proteomes, and specifically, the formation of trioxidized Cys (t-Cys), which leads to permanent protein damage. Recent findings in rodents have uncovered that irregular regulation of t-Cys residues in the aging proteome disrupts homeostatic phosphorylation signaling, resulting in alterations to proteins that are analogous to those caused by phosphorylated serine (p-Ser) residues. This work contextualizes these significant findings and discusses the implications and molecular role(s) of t-Cys in the aging proteome. Furthermore, we present novel data, validating the increase of specific t-Cys sites associated with aging in a blood-related circulating human proteome. The scope and findings included here support the hypothesis that t-Cys residues may serve as important mechanistic and biological markers, warranting further exploration in the context of unhealthy aging and age-related major diseases.