A vehicle body rear part structure includes a bumper beam, an extension, and a gusset. The bumper beam extends in a vehicle width direction in a rear part of a vehicle body. The extension is provided for attaching the bumper beam to the vehicle body. The gusset is provided between the bumper beam and the extension and at an inner position in a vehicle width direction of the extension. A deformation facilitating part is provided on a middle part of the gusset.