A developer supply cartridge detachably mountable to an image forming apparatus, includes: a developer accommodator accommodating a developer; a discharging opening discharging the developer accommodated in the accommodator to a developer receiving opening of a developer receiving cartridge developing an electrostatic image formed on an image bearing member with the developer, and being detachably mountable to the image forming apparatus; a shutter movable between an opening position opening the discharging opening and a closing position closing the discharging opening; an engager engaging the developer receiving cartridge to receive, from the developer receiving cartridge, a force for moving the shutter from the closing to the opening position, in interrelation with a relative movement between the developer supply cartridge and the developer receiving cartridge toward each other in a mounting-and-demounting direction of the developer supply cartridge relative to the image forming apparatus.