A vehicle side section structure including: a center pillar including an outer panel, an inner panel that forms a closed cross-section with the outer panel, and a reinforcement member that is provided inside the closed cross-section, that reinforces the outer panel, and that includes a main body section including a front wall, a side wall, and a rear wall and open toward the vehicle width direction inside, and a recessed portion formed between a front side ridgeline formed by the front wall and the side wall and a rear side ridgeline formed by the side wall and the rear wall and open toward the vehicle width direction outside; and filler material filled between the recessed portion and a facing face that faces the recessed portion, excluding at the periphery of the front side ridgeline and at the periphery of the rear side ridgeline of the main body section.