A vehicle side portion structure includes: an outer member; an inner member disposed closer to a center of a vehicle than the outer member in a vehicle width direction and facing the outer member, the inner member partially fixed to the outer member, the inner member and the outer member forming a tubular shape having a space; a main wall portion provided in the space, the main wall portion extending along the outer member, the main wall portion being spaced from the outer member, an acceleration sensor fixed to the main wall portion provided with a projecting portion, the projecting portion projecting to be further from the center of the vehicle than a fixing position where the acceleration sensor is fixed in the vehicle width direction; and a fixing portion connected to the main wall portion, the fixing portion being configured to be fixed to the outer member.