A small-area side-capacitor read-only memory device, a memory array and a method for operating the same are provided. The small-area side-capacitor read-only memory device embeds a field-effect transistor in a semiconductor substrate. The field-effect transistor includes a first dielectric layer and a first conductive gate stacked on the first dielectric layer. The side of the first conductive gate extends to the top of the second dielectric layer and connects to the second conductive gate to generate a capacitance effect. The second conductive gate has finger portions connected to a strip portion. Thus, the memory device employs the smallest layout area to generate the highest capacitance value, thereby decreasing the overall area of the read-only memory and performing efficient reading and writing.