Background and PurposeInflammatory reaction plays a critical role in the pathogenesis of late delayed radiation‐induced brain injury (RBI). Low vitamin D levels are closely associated with various immuno‐inflammatory diseases, but the relationship with late delayed RBI remains unknown. Here, we aimed to determine the association of serum vitamin D levels with clinical parameters in late delayed RBI patients with nasopharyngeal carcinoma.Methods25‐Hydroxyvitamin D3 levels and clinical and cerebrospinal fluid parameters were evaluated in 21 patients with RBI and compared with 90 age‐, sex‐, and season‐matched healthy controls.Results25‐(OH)D3 levels were lower in patients with RBI compared to controls (40.39 ± 22.11 vs. 64.54 ± 19.89 nmol/L, p < .001), especially for aged ≥60 years (vs. <60 years, p = .038), females (vs. males, p = .012), short latency (<5 years) (vs. ≥5 years, p = .015), and severe impairment (LENT/SOMA score ≥3) (vs. LENT/SOMA score <3, p = .010). Serum 25‐(OH)D3 levels were associated with age (r = −.464, p = .015), Latency of RBI (r = .416, p = .031) and LENT/SOMA Scale (r = −.488, p = .010).ConclusionsOur data showed that serum 25‐(OH)D3 levels were reduced in late delayed RBI patients with nasopharyngeal carcinoma.