An input/output (I/O) buffer circuit includes an I/O unit and a protection circuit. The I/O unit selectively receives and outputs signals based on an enable signal. The protection circuit generates a logic control signal to deactivate the I/O unit in a state where a voltage level of the I/O terminal is abnormal. The protection circuit includes a register. The register latches a logic signal corresponding to the voltage level of the I/O terminal in a state where the voltage level of the I/O terminal is abnormal, outputs the logic control signal based on the logic signal, and is preset to output the logic control signal based on the logic signal when a power-off state resumes to a power-on state.