Ataxia-telangiectasia (A-T) is a multisystem disorder in which nutritional and endocrine abnormalities are frequently reported. Vitamin D deficiency has been described in A-T cohorts, but its pooled prevalence within published A-T cohorts has not been quantified. We conducted a systematic review and single-arm meta-analysis to estimate the prevalence of vitamin D deficiency in A-T. We systematically identified observational studies reporting serum 25-hydroxyvitamin D 25(OH)D in individuals with A-T. The primary outcome was vitamin D deficiency prevalence using the study-defined threshold of 25(OH)D < 20 ng/mL. Secondary outcomes included severe deficiency (< 10 ng/mL), insufficiency, adequacy, and pooled mean 25(OH)D where available. Random-effects single-arm meta-analyses were performed. Study quality was assessed using the Newcastle–Ottawa Scale. Four studies were included. The pooled prevalence of vitamin D deficiency (< 20 ng/mL) was 50.8% (95% CI 39.4% to 62.2%), with I² = 0%. The pooled prevalence of severe deficiency (< 10 ng/mL) was 26.8% (95% CI 10.4% to 43.1%), with I² = 23%. The pooled prevalence of insufficiency was 21.9% (95% CI 7.6% to 36.1%), with I² = 0%, and adequacy was 38.1% (95% CI 19.5% to 56.8%), with I² = 55.1%. The pooled mean 25(OH)D was 19.65 ng/mL (95% CI 16.26 to 23.04), with I² = 30.7%. Vitamin D deficiency is common in published A-T cohorts, with approximately half meeting a < 20 ng/mL threshold. These findings support routine clinical attention to 25(OH)D in A-T, while highlighting the need for standardized, outcome-focused prospective studies and interventional trials. Graphical Abstract