There is provided a vehicle side portion joining portion structure capable of promoting a reduction in weight of a vehicle body and of effectively suppressing vibrations and deformations of a pillar, regardless of structures of the pillar and a rocker and the like. A pillar reinforcement member is joined to an interior of a foot portion of a pillar outer that is formed with an open cross section in which a vehicle width direction inner side is opened. At a vehicle forward side of the foot portion, a front side closed cross section is formed at a vehicle front side portion of the foot portion. At a vehicle rearward side of the foot portion, a rear side closed cross section is formed at a vehicle rear side portion of the foot portion.