The aim of this study was to investigate the relationship between tempromandibular disorders (TMDs) and nail biting among high school students in Riyadh, Saudi Arabia. Methods: Five high schools in Riyadh were selected from five different regions to collect Randomly 424 students divided into two groups; 224 students with nail biting habit in the first group, and 200 students who does not have that habit in the second group. A questionnaire and TMJ examination were performed on both groups. Results: The most common symptoms of TMDs in the first group was pain in the TMJ (23.2%) (p<0.000), with emotional stresses reached (23.9%) (p<0.000) , while the most common signs found in the experimental group were tenderness in the masseter muscle (26.8%) (p<0.000), and pain upon posterior palpation of TMJ capsule (46.4%) (p<0.000). Conclusion: The results shows a strong influence of nail biting on TMDs, so it is important to increase the level of awareness about the risks of nail biting habit among teenagers and parents.