Purpose: This study aims to determine the prevalence of orthorexia nervosa (ON) among university students and to evaluate the relationship between stress and ON, as well as the effects that ON may have on the health of these individuals. Methods: In this cross-sectional study, a total of 205 participants (66.7% women) were recruited through informational posters on the university campus during the 2022–2023 academic year. They answered different questionnaires to yield socio-demographic data and completed specific tests for the evaluation of ON (Düsseldorf Orthorexia Scale (DOS-ES), Eating Habits Questionnaire (EHQ-ES)) and stress (Perceived Stress Scale (PSS-ES)). The analytical determination of blood biomarkers was also carried out. Results: The prevalence of ON obtained from the DOS-ES questionnaire was 1.5%, while 7.5% of the individuals showed a risk of ON. In addition, a positive correlation was observed between DOS-ES and EHQ-ES scores (rs = 0.674). A weak correlation (rs = 0.138) was reported between stress and ON. Individuals with underweight BMI (OR: 1.11, 95% CI: 1.01–1.22) and elevated monocyte levels (OR: 1.15, 95% CI: 1.05–1.26) were more likely to have higher DOS-ES scores compared to those with normal weight and normal monocyte levels. Conclusions: Our study demonstrated a lower rate (1.5%) than previous studies, and differences by sex or age were not observed in ON diagnosis, nor was a link between underweight BMI and an increased risk of ON. Additionally, a higher monocyte count was associated with ON, suggesting potential immune and cardiometabolic implications, but further research with larger populations is needed to confirm these findings.