<p>Serpins, characterized by a conserved structural fold, serve diverse biological roles. Protein Z-dependent protease inhibitor (ZPI), a serpin superfamily member, acts as an endogenous anticoagulant by inhibiting clotting factors Xa (fXa) and XIa (fXIa). Beyond anticoagulation, ZPI has roles in inflammation, cancer, and immune regulation. However, its exact pathophysiological role is yet to be fully characterized. To elucidate ZPI’s evolutionary trajectory and non-haemostatic roles, we conducted a comprehensive phylogenetic analysis integrating sequence, gene structure, and synteny data. We identified a lamprey-specific serpin, ZPIL_AGTL_PMA, containing both an inhibitory reactive center loop (RCL) and an angiotensin II (Ang II) motif. This finding suggests that ZPIL_AGTL_PMA represents an ancestral bifunctional serpin from which ZPI and angiotensinogen (AGT), a non-inhibitory serpin involved in blood pressure regulation, evolved by sub-functionalization in jawed vertebrates. This bifunctionality within a single gene in lamprey likely reflects an ancestral vertebrate trait. Gene cluster analyses showed serpinA10 (ZPI) as possibly the earliest member, with other Clade A serpins arising via subsequent duplication. The chromosomal location of this gene cluster is conserved in most vertebrates, except Carnivores and Suidea. Sequence analysis indicated potential non-inhibitory ZPI variants in certain species with atypical non-serine residues at the P1′ position within its RCL, a critical determinant of inhibitory serpin activity. The close evolutionary relationship between ZPI and AGT further suggests mechanistic interplay between coagulation and blood pressure regulation, highlighting shared regulatory pathways involving these serpins. Together, these findings expand the functional landscape of ZPI and underscore the dynamic evolution of serpin-mediated physiological processes.</p>