Naturalness is a key concept in research on landscape preference and restorative environments. However, what it is that constitutes the naturalness and the natural qualities has been less studied and the physical attributes identified are still fuzzy. This study suggests that fractal geometry, which is abundant in nature, could be used as a tool for defining perceived naturalness. The results show that landscape silhouette outlines with a fractal dimension around 1.3 were rated as most natural indicating that this particular fractal dimension may play an important role in people’s definition of perceived naturalness. Similarly, outlines with fractal dimension around 1.3 were rated as most preferred. The study used data from both psychology students and landscape architectural students and the results show that naturalness judgements were less affected by subjects’ background than preference.