Integrated circuit devices may include a substrate including a flash memory region and a variable resistance memory region, a flash memory cell transistor including a cell gate electrode that overlaps the flash memory region of the substrate, a variable resistance element that overlaps the variable resistance memory region of the substrate, and a select transistor including a select source/drain region that is disposed in the variable resistance memory region of the substrate. The select source/drain region may be electrically connected to the variable resistance element. The substrate may include an upper surface facing the cell gate electrode and the variable resistance element, and the upper surface of the substrate may continuously extend from the flash memory region to the variable resistance memory region.