Semiconductor devices and methods of forming the same are provided. Semiconductor devices may include a substrate including first and second regions, first active fins extending in a first direction on the first region, second active fins extending parallel to the first active fins on the second region, and single diffusion break regions between two first active fins. Single diffusion break regions may be spaced apart from each other in the first direction. The semiconductor devices may also include a lower diffusion break region between two second active fins and extending in a second direction that is different from the first direction and upper diffusion break regions on the lower diffusion break region. The upper diffusion break regions may be spaced apart from each other in the first direction, and each of the upper diffusion break regions may overlap the lower diffusion break region.