A nonvolatile memory device, which has an improved read reliability through a refresh operation, and a memory system, are provided. The nonvolatile memory device includes a resistive memory cell, a reference resistor corresponding to the resistive memory cell, a reference sense amplifier electrically connected to the reference resistor and configured to change a transition time of an output value of the reference resistor, and a refresh request signal generator configured to output the refresh request signal for the resistive memory cell when the transition time of an output value of the reference resistor is in a preset refresh requiring period.