Conventional photodynamic therapy (PDT) for cancer is limited by the insufficient efficacy and specificity of photosensitizers. We herein describe a highly effective and selective tumor‐targeted PDT using a near‐infrared (NIR) photosensitizer, IRDye700DX, conjugated to a human monoclonal antibody (Ab) specific for carcinoembryonic antigen (CEA). The antitumor effects of this Ab‐assisted PDT, called photoimmunotherapy (PIT), were investigated in vitro and in vivo. The Ab‐IRDye conjugate induced potent cytotoxicity against CEA‐positive tumor cells after NIR‐irradiation, whereas CEA‐negative cells were not affected at all, even in the presence of excess photoimmunoconjugate. We found an equivalent phototoxicity and a predominant plasma membrane localization of Ab‐IRDye after both one and six hours of incubation. Either no or little caspase activation and membrane peroxidation were observed in PIT‐treated cells and a panel of scavengers for reactive oxygen species showed only partial inhibition of the phototoxic effect. Strikingly, Ab‐IRDye retained significant phototoxicity even under hypoxia. We established a xenograft model, which allowed us to sensitively investigate the therapeutic efficacy of PIT by non‐invasive bioluminescence imaging. Luciferase‐expressing MKN‐45‐luc human gastric carcinoma cells were subcutaneously implanted into both flanks of nude mice. NIR‐irradiation was performed for only the tumor on one side. In vivo imaging and measurement of the tumor size revealed that a single PIT treatment, with intraperitoneal administration of Ab‐IRDye and subsequent NIR‐irradiation, caused rapid cell death and significant inhibition of tumor growth, but only on the irradiated side. Together, these data suggest that Ab‐IRDye‐mediated PIT has great potential as an anticancer therapeutics targeting CEA‐positive tumors.