Glycoconjugated chlorins represent a promising class of compounds that meet the requirements for the third-generation photosensitizer (PS) for photodynamic therapy (PDT). We have focused on the use of glucose (Glc) to improve the performance of the PS based on the Warburg effect—a phenomenon where tumors consume higher Glc levels than normal cells. However, as a matter of fact, Glc-conjugation has a poor efficacy in hydrophilic modification; thus, the resultant PS is not suitable for intravenous injection. In this study, a Glc-based oligosaccharide, such as maltotriose (Mal3), is conjugated to chlorin e6 (Ce6). The conjugation is assisted by two additional molecular tools, such as propargyl amine and a tetraethylene glycol (TEG) derivative. This route produced the target Mal3–Ce6 conjugate linked via the TEG spacer (Mal3–TEG–Ce6), which shows the required photoabsorption properties in the physiological media. The PDT test using canine mammary carcinoma (SNP) cells suggested that the antitumor activity of Mal3–TEG–Ce6 is extremely high. Furthermore, in vitro tests against mouse mammary carcinoma (EMT6) cells have been demonstrated, providing insights into the photocytotoxicity, subcellular localization, and analysis of cell death and reactive oxygen species (ROS) generation for the PDT system with Mal3–TEG–Ce6. Both apoptosis and necrosis of the EMT6 cells occur by ROS that is generated via the photochemical reaction between Mal3–TEG–Ce6 and molecular oxygen. Consequently, Mal3–TEG–Ce6 is shown to be a PS showing the currently desired properties.