Objective: To examine vitamin D levels in patients diagnosed with fibromyalgia syndrome (FMS). Methods: This study was performed at Riyadh Care Hospitals, Saudi Arabia. The data were obtained from rheumatology outpatient clinic records that routinely measure Vitamin D levels. The total number of patients in this database was 1,072. The gathered data included vitamin D serum concentration, age, and gender, as well as body mass index (BMI). Patients were diagnosed according to Wolfe, et al. criteria for diagnosing FMS. Vitamin D deficiency (VDD) was defined according to Mayo Medical Laboratories Reference Ranges for Total Serum 25-hydroxyvitamin D [25(OH)D]: Optimal: 25-80 ng/mL; mild to moderate: 10-24 ng/mL; severe: < 10 ng/mL. Results: Twenty-eight patients were included in the study. The mean age was 37.8 ± 7.0 years. The average BMI was 29.3 ± 4.22. Mean Vitamin D level was 10.23 ± 4.99 ng/ mL. The overall prevalence rate of VDD (i.e., 24 ng/mL and below) was high at 96.4% (27/28). In terms of severity, most of the patients (20/28; %) had severe VDD (i.e., below 10 ng/mL) with a prevalence rate of 71.4%. Regression analyses demonstrated that FMS patients with older age (45 and above), and higher BMI (30 and above) had a higher probability of having severe VDD. Conclusion: The condition of VDD is frequently seen in patients diagnosed with FMS. Patients with FMS, old age, and high BMI had a higher probability of having severe VDD. Although the sample size of the study is small, the figures are alarming, suggesting that practitioners' need to be vigilant regarding the testing of vitamin D serum levels in this patient population. Screening for VDD is particularly critical for patients 45 years and older or with BMI that is 30 and above.