A vehicle body lower portion structure includes: a lower frame member formed as a chamber that extends in a vehicle body longitudinal direction, having a guide portion at inner surfaces of wall portions that structure the chamber, the lower frame member being disposed at a vehicle body lower side; and a reinforcing member formed as a second chamber that extends in the vehicle body longitudinal direction, having a plurality of projections at outer surfaces of wall portions that structure the second chamber, the reinforcing member being provided within the chamber of the lower frame member in a state in which the plurality of projections are made to contact the guide portion.