Embodiments of the inventive concept provide a nonvolatile memory device. The nonvolatile memory device includes a memory cell array, a read/write circuit, and a backup circuit. The memory cell array includes a first memory block including a first word line having first memory cells and a second word line having second memory cells. Each of the first memory cells and second memory cells configured to store first-bit data and second-bit data. The read/write circuit is configured to program data into the first and second memory cells and read data stored in the first and second memory cells. The backup circuit is configured to, after first-bit data are programmed into the first word line, but before second-bit data are programmed into the first word line, store first-bit data stored in the second memory cells of the second word line.