Serpin is an important member of serine protease inhibitors (SPIs), which is capable of regulating proteolytic events and involving in a variety of physiological processes. In present study, a Serpin homolog was identified from Octopus ocellatus (designated as OoSerpin). Full-length cDNA of OoSerpin was of 1735 bp, containing a 5′ untranslated region of 214 bp, a 3′ UTR of 282 bp, and an open reading frame of 1239 bp. The open reading frame encoded a polypeptide of 412 amino acids which has a predicted molecular weight of 46.5 kDa and an isoelectric point of 8.52. The OoSerpin protein shares 37% sequence identity with other Serpins from Mus musculus (NP_941373) and Ixodes scapularis (XP_002407493). The existence of a conserved SERPIN domain strongly suggested that OoSerpin was a member of the Serpin subfamily. Expression patterns of OoSerpin, both in tissues and towards bacterial stimulation, were then characterized. The mRNA of OoSerpin was constitutively expressed at different levels in all tested tissues of untreated O. ocellatus, including mantle (lowest), muscle, renal sac, gill, hemocyte, gonad, systemic heart, and hepatopancreas (highest). The transcriptional level of OoSerpin was significantly up-regulated (P < 0.01) in O. ocellatus upon bacterial challenges with Vibrio anguillarum and Micrococcus luteus, indicating its involvement in the antibacterial immune response. Furthermore, rOoSerpin, the recombinant protein of OoSerpin, exhibited strong abilities to inhibit proteinase activities of trypsin and chymotrypsin as well as the growth of Escherichia coli. Our results demonstrate that OoSerpin is a potential antibacterial factor involved in the immune response of O. ocellatus against bacterial infection.