A semiconductor device includes first group active fins and a first diffusion prevention pattern. The first group active fins are spaced apart from each other in a second direction, and each of the first group active fins extends in a first direction different from the second direction on a first region of a substrate including the first region and a second region. The first diffusion prevention pattern extends on the first region of the substrate in the second direction through the first group active fins. The first group active fins include first and second active fins. The first diffusion prevention pattern extends through a central portion of the first active fin in the first direction to divide the first active fin, and extends through and contacts an end of the second active fin in the first direction.