In cancer photodynamic therapy (PDT), a photosensitizer taken up by cancer cells can generate reactive oxygen species upon near-infrared light activation to induce cancer cell death. To increase PDT potency and decrease its adverse effect, one approach is to conjugate the photosensitizer with an antibody that specifically targets cancer cells. In the present study, IR700, a hydrophilic phthalocyanine photosensitizer, was conjugated to the humanized monoclonal antibody ARB102, which binds specifically cadherin-17 (CDH17 aka CA17), a cell surface marker highly expressed in gastrointestinal cancer to produce ARB102-IR700. Photoimmunotherapy (PIT) of gastrointestinal cancer cell lines was conducted by ARB102-IR700 treatment and near-infrared light irradiation. The results showed that ARB102-IR700 PIT could induce cell death in CDH17-positive cancer cells with high potency. In a co-culture model, CDH17-negative and CDH17-overexpressing SW480 cells were labeled with distinct fluorescent dyes and cultured together prior to PIT treatment. The results confirmed that ARB102-IR700 PIT could kill CDH17-positive cells specifically, while leaving the adjacent CDH17-negative cells unaffected. An in vivo efficacy study was conducted using a pancreatic adenocarcinoma AsPC-1 xenograft tumor model in nude mice. Fluorescence scanning indicated that ARB102-IR700 accumulated specifically in the tumor sites. To perform PIT, at 24 and 48 h postinjection, mice were irradiated with a 680 nm laser at the tumor site to activate the photosensitizer. It was shown that ARB102-IR700 PIT could inhibit tumor growth significantly. In summary, this study demonstrated that the novel ARB102-IR700 is a promising agent for PIT in gastrointestinal cancers.