A vehicle rear portion structure comprising: an upper back; an upper back reinforcement made of a material different from the upper back, the upper back reinforcement being provided to extend in a vehicle width direction on an upper side of a rear portion of the upper back in a vehicle front-rear direction, wherein: the upper back reinforcement includes a front flange, a rear flange behind the front flange in the vehicle front-rear direction, and a bulging portion provided to bulge upward between the front flange and the rear flange; and a luggage opening flange provided to rise upward from a rear end portion of the rear flange in the vehicle front-rear direction and form an opening edge on a front side of a luggage room; and a luggage opening sealing strip, provided continuously over an entire circumference of the opening edge of the luggage room, and attached to the luggage opening flange; wherein the upper back is joined to the front flange of the upper back reinforcement at an intermediate portion of the upper back in the vehicle front-rear direction by any of caulking joining, bolt joining, and rivet joining and is joined to the rear flange of the upper back reinforcement at a rear end portion of the upper back in the vehicle front-rear direction; and a rear side joining portion as a joining portion between the upper back and the rear flange of the upper back reinforcement is covered with the luggage opening sealing strip. an upper back; an upper back reinforcement made of a material different from the upper back, the upper back reinforcement being provided to extend in a vehicle width direction on an upper side of a rear portion of the upper back in a vehicle front-rear direction, wherein: the upper back reinforcement includes a front flange, a rear flange behind the front flange in the vehicle front-rear direction, and a bulging portion provided to bulge upward between the front flange and the rear flange; and a luggage opening flange provided to rise upward from a rear end portion of the rear flange in the vehicle front-rear direction and form an opening edge on a front side of a luggage room; and a luggage opening sealing strip, provided continuously over an entire circumference of the opening edge of the luggage room, and attached to the luggage opening flange; wherein the upper back is joined to the front flange of the upper back reinforcement at an intermediate portion of the upper back in the vehicle front-rear direction by any of caulking joining, bolt joining, and rivet joining and is joined to the rear flange of the upper back reinforcement at a rear end portion of the upper back in the vehicle front-rear direction; and a rear side joining portion as a joining portion between the upper back and the rear flange of the upper back reinforcement is covered with the luggage opening sealing strip. the upper back reinforcement includes a front flange, a rear flange behind the front flange in the vehicle front-rear direction, and a bulging portion provided to bulge upward between the front flange and the rear flange; and a luggage opening flange provided to rise upward from a rear end portion of the rear flange in the vehicle front-rear direction and form an opening edge on a front side of a luggage room; and a front flange, a rear flange behind the front flange in the vehicle front-rear direction, and a bulging portion provided to bulge upward between the front flange and the rear flange; and a luggage opening flange provided to rise upward from a rear end portion of the rear flange in the vehicle front-rear direction and form an opening edge on a front side of a luggage room; and a luggage opening sealing strip, provided continuously over an entire circumference of the opening edge of the luggage room, and attached to the luggage opening flange; wherein the upper back is joined to the front flange of the upper back reinforcement at an intermediate portion of the upper back in the vehicle front-rear direction by any of caulking joining, bolt joining, and rivet joining and is joined to the rear flange of the upper back reinforcement at a rear end portion of the upper back in the vehicle front-rear direction; and a rear side joining portion as a joining portion between the upper back and the rear flange of the upper back reinforcement is covered with the luggage opening sealing strip.