Introduction: Despite abundant sunshine for cutaneous vitamin D synthesis, low levels of vitamin D have been documented among the Malaysian population. The aim of this study was to characterise the effects of occupational sunlight exposure and monsoon season on serum 25-hydroxyvitamin D [25(OH)D] concentration. Methods: A comparative cross-sectional study was performed among Malay outdoor (n=119) and indoor workers (n=119) in Kelantan. Two-point data were collected on the same participants, first during non-monsoon season and second during the northeast monsoon season. Data collection comprised of anthropometry measurements (body mass index and body fat), fasting blood test [serum 25(OH)D concentration], and questionnaire (physical activity level, sun exposure, sun protection use, and vitamin D intake). Vitamin D classification was based on the 2011 Endocrine Society Clinical Practice Guidelines. Results: This study found that serum 25(OH)D concentrations were significantly higher in outdoor compared to indoor workers irrespective of season (p<0.001) and sex (p<0.001). Monsoonal differences in serum 25(OH)D concentration was only observed in male outdoor workers (mean difference=10.39 nmol/l, p<0.001). Significant association between vitamin D status and occupation was also observed (p<0.001). However, no significant association was found between vitamin D status and monsoon season [χ2(1)=0.076, p=0.783]. Conclusion: Seasonal and occupational factors should be considered while evaluating individual serum 25(OH)D concentration and in comparing community studies, especially among workers.