A toner cartridge includes a casing provided at a bottom side thereof with an outlet, a sliding member slidably mounted on the casing, and a retaining member pivotally mounted on the casing. The sliding member is movable relative to the casing between a first position where the outlet is closed by the sliding member and a second position where the outlet is opened. The retaining member is pivotable relative to the casing between a third position where the retaining member is engaged with the sliding member to lock the sliding member at the first position, and a forth position where the retaining member is disengaged from the sliding member such that the sliding member is moveable to the second position to open the outlet. The toner cartridge has the advantage of preventing leakage of toner due to unintentional movement of the sliding member.