Photodynamic therapy (PDT) utilizes photochemical reactions induced by a photosensitizer and light in the target tissue and is used to treat various cancers. There is a high degree of anticipation of success regarding the application of PDT with talaporfin (photosensitizer) for gastric cancer. Olaparib is an oral inhibitor of Poly (ADP-Ribose) polymerase (PARP) and has demonstrated optimal efficacy and clinical activity in trials. Therefore, the aim of the present study was to investigate the efficacy of talaporfin PDT combined with olaparib for gastric cancer. MKN45, a gastric cancer cell line, was incubated with talaporfin, followed by irradiation, in the presence/absence of olaparib. Talaporfin PDT and olaparib exhibited excellent synergistic action in a concentration-dependent manner. PARP-DNA complexes were characterized based on bound chromatin using Western blot analyses. The combination of talaporfin PDT and olaparib enhanced PARP1 accumulation (the entrapment of PARP1-DNA complexes) in bound chromatin. The combination of talaporfin PDT and olaparib induced DNA double-strand breaks, which was confirmed by evaluating phosphorylated histone H2AX. Xenograft tumor mouse models were established, and antitumor effects were analyzed. In vivo, tumor growth was significantly suppressed following PDT with talaporfin and olaparib. Our results demonstrated that olaparib enhances the efficacy of talaporfin PDT by inducing the formation of PARP-DNA complexes. Therefore, our results suggest that the combination of talaporfin PDT and olaparib is a potential antitumor therapy for gastric cancer. (C) 2020 Elsevier Inc. All rights reserved.