A portable toothbrush sterilizer is provided. The portable toothbrush sterilizer includes a lower case (100a), an upper case (200a), a protective cap (500), and a cover (300a). The lower case (100a) has a sterilizer compartment (150) and a battery compartment (140). The upper case (200a) is coupled above the lower case (100a) and includes a storage space (210) and a switch hole (250) with a touch switch (164) therein. The storage space (210) has a toothbrush bristle compartment (212) with an ultraviolet ray emitting hole (240) formed on a side thereof and a toothbrush handle compartment (214). The protective cap (500) is detachably attached to the toothbrush bristle compartment (212) and includes a second ultraviolet ray emitting hole (510) and an open portion (550). The cover 300a is disposed above the upper case (200a) and includes a button (340) for pressing the touch switch (164).