INTRODUCTION: Vitamin D deficiency is commonly found among patients with obstructive sleep apnea (OSA). We aimed to determine the effect of an eight-week interdisciplinary weight loss and lifestyle intervention on circulating vitamin D levels in patients with moderate-to-severe OSA. METHODS: 89 men were assigned to a usual-care group (n = 49) or an 8-week interdisciplinary weight loss and lifestyle intervention combined with usual-care (n = 40). Evaluations were conducted at baseline, intervention endpoint (i.e., 8 weeks), and 6 months post-intervention. Serum 25-hydroxyvitamin D (25(OH)D) was determined using a chemiluminescence immunoassay. Sleep (i.e., sleep efficiency, apnea-hypopnea index [AHI], and oxygen desaturation index) and body weight and composition (i.e., fat mass, and visceral adipose tissue) variables were also determined. RESULTS: Serum 25(OH)D concentrations showed an insufficient vitamin D status at baseline, which significantly increased (all p ≤ 0.034) at intervention endpoint (19 %) and at 6 months after intervention (45 %) in the intervention group to the point of potentially resolving vitamin D deficiency. Higher serum 25(OH)D concentrations were related to increased sleep efficiency and reduced AHI, oxygen desaturation index, and body weight and composition variables (all p <
0.002) from baseline to 6 months and from 8 weeks to 6 months after intervention. These results were also noted from baseline to 8 weeks, except for body composition (all p ≤ 0.007). CONCLUSION: The intervention improved and potentially resolved vitamin D deficiency. Together with the improvement of adverse sleep patterns and body composition parameters, it may be considered as a promising approach in the treatment of OSA. CLINICAL TRIAL REGISTRATION: (ClinicalTrials.gov NCT03851653).