Background/Objectives: Vitamin D deficiency is widespread globally, including in the Middle East. In the UAE, vitamin D deficiency contributes to 78% of bone losses because of cultural and lifestyle factors, which limit sun exposure. Although supplementation is effective, increasing rates of self-prescribed use raise concerns about safety and efficacy, particularly among university students. Therefore, this study aims to assess the prevalence, patterns, and predictors of self-prescribed vitamin D supplementation among university students in the United Arab Emirates. Methods: A descriptive cross-sectional survey was conducted among 450 university students aged 18–39 who had used vitamin D supplements in the past 12 months. Data were collected using an online questionnaire and analyzed using chi-square tests and binary logistic regression. Results: Among participants, 44.9% reported self-prescribed vitamin D use. Males were more likely to self-prescribe (p = 0.010). Self-prescribers used supplements for shorter durations (p < 0.001) and were more likely to report motivations like physical performance (p = 0.005). Predictors of self-prescription included short-term use (OR = 2.57), non-daily intake (OR = 3.49), use for performance (OR = 2.72), and concurrent vitamin C use (OR = 1.85). Conclusions: Self-prescription of vitamin D is common among university students and associated with irregular use and non-clinical motivations, primarily to improve overall health and wellness. While such practices are unlikely to result in toxicity, they may not adequately address the widespread problem of vitamin D deficiency. These findings suggest the need for clear, locally relevant guidance to promote safe and effective supplementation among young adults.