The mental health of university undergraduates is a critical concern globally, with challenges faced by students in unique academic environments. This study was conducted at General Sir John Kotelawala Defence University to assess the prevalence of mental health issues and identify associations with participant characteristics. A descriptive cross-sectional study involving 640 allied health sciences undergraduates was performed. The Depression, Anxiety, and Stress Scale-21 Items (DASS-21) was utilized for evaluation. Statistical analysis was performed with SPSS 23.0, and associations were explored using Spearman's correlation and Pearson's chi-square tests. Ethical approval was obtained, ensuring participant confidentiality and informed consent. The study revealed mild to extremely severe symptoms of depression (35.1%), anxiety (40.5%), and stress (27.7%) among the participants. Strong positive relationships were identified between depression and anxiety (r=0.707, p=.000), depression and stress (r=0.722, p=.000), and anxiety and stress (r=0.658, p=.000). Significant associations were found between mental health levels and participants currently receiving medical treatments and those with a history of psychological treatments. While one-third of the participants exhibited concerning levels of mental health symptoms, our study indicated comparatively lower rates than did previous research in Sri Lanka. The findings highlight the need to enhance mental health support and accessibility of services for university undergraduates. The study contributes valuable insights for future research and interventions, emphasizing the unique challenges faced by students in a defence university setting.