Acne is a skin inflammatory disorder that affects a large number of people. Vitamin D has been linked to a variety of dermatological conditions, including psoriasis, atopic dermatitis, and acne vulgaris, according to recent research. Vitamin D is involved in immune system modulation as well as the proliferation and differentiation of sebocytes and keratinocytes. Previous studies in Asia revealed vitamin D deficiency was more frequent in acne subjects compared to healthy subjects. However, there is no available data on the vitamin D level among Thai acne patients. This study aims to determine 25-hydroxyvitamin D (25(OH)D) levels in acne-diagnosed participants. The study was conducted at Benchakiti Park hospital, Thailand in June 2021. Twenty patients with a diagnosis of acne were examined for acne severity using the global acne grading system (GAGS) and classified the acne severity into four groups: mild, moderate, severe, and very severe. Serum 25(OH)D levels were measured. Our results demonstrated that none of the acne patients reached a sufficient level of vitamin D. The mean 25(OH)D level was 16.85 ± 4.38 ng/mL. There was no significant correlation between 25-hydroxyvitamin D levels and acne severity. This finding showed that low vitamin D level is very common among acne patients.  The study's limitations include a limited number of individuals and a lack of patients with severe and very severe acne.