Chlorophyll degradation plays important roles in leaf senescence including regulation of degradation of chlorophyll-binding proteins. Although most genes encoding enzymes of the chlorophyll degradation pathway have been identified, the regulation of their activity has not been fully understood. Green cotyledon mutants in legume are stay-green mutants, in which chlorophyll degradation is impaired during leaf senescence and seed maturation. Among them, the soybean (Glycine max) green cotyledon gene cytG is unique because it is maternally inherited. To isolate cytG, we extensively sequenced the soybean chloroplast genome, and detected a 5-bp insertion causing a frame-shift in psbM, which encodes one of the small subunits of photosystem II. Mutant tobacco plants (Nicotiana tabacum) with a disrupted psbM generated using a chloroplast transformation technique had green senescent leaves, confirming that cytG encodes PsbM. The phenotype of cytG was very similar to that of mutant of chlorophyll b reductase catalyzing the first step of chlorophyll b degradation. In fact, chlorophyll b-degrading activity in dark-grown cytG and psbM-knockout seedlings was significantly lower than that of wild-type plants. Our results suggest that PsbM is a unique protein linking photosynthesis in presenescent leaves with chlorophyll degradation during leaf senescence and seed maturation. Additionally, we discuss the origin of cytG, which may have been selected during domestication of soybean.