Crymlyn Bog (Welsh: Cors Crymlyn) is a nature reserve and a designated Site of Special Scientific Interest of international significance, near Swansea, south Wales. It is the largest area of lowland fen in Wales and lies immediately to the eastern side of Kilvey Hill just north of the industrial area of Crymlyn Burrows. There is a visitor centre in the bog, which is frequented by nature and bird watching enthusiasts.