A developer container includes: a casing having a filling port and a cap for covering the filling port. The filling port is defined by a cylindrical through-hole having an inner peripheral surface, and a protruding portion protruding radially inward from the inner peripheral surface. The cap is attached to the casing in a first direction. The cap includes: a main body portion in a tubular shape having an outer peripheral surface; and a flange portion protruding radially outward from the outer peripheral surface in annular fashion. The flange portion is positioned downstream of the main body portion in the first direction and configured to be in contact with the inner peripheral surface. The protruding portion is positioned upstream of the flange portion in the first direction upon attachment of the cap to the casing.