A bicycle battery pack that can be reduced in volume includes a housing configured to be located on a frame of a bicycle and battery cells accommodated in the housing. The battery cells include at least a first battery cell and a second battery cell. The housing includes a side wall extending in a longitudinal direction of the frame in a state located on the frame. An outer portion of the side wall includes at least one groove. The at least one groove is recessed toward a region between the first battery cell and the second battery cell.