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Objectives To investigate the impact of vitamin D on disease activity, function and health in patients with radiographic axial spondyloarthritis (r-axSpA) undergoing biological disease-modifying antirheumatic drug (bDMARD) therapy.Methods Patients with r-axSpA and active disease at baseline initiating a bDMARD were included in this analysis. Vitamin D (25-hydroxyvitamin D) was measured at baseline and every 6 months until year 2; deficiency was defined as <20 ng/mL. The outcomes were AxSpA Disease Activity Score (ASDAS), Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), Patient Global Assessment (PGA), Bath Ankylosing Spondylitis Functional Index (BASFI), Assessment of SpondyloArthritis international Society Health Index (ASAS-HI) and C reactive protein (CRP). Longitudinal associations were evaluated using generalised estimating equations, with sensitivity analyses accounting for time-varying confounders.Results We analysed 122 patients (mean age 36.6 (10.3) years; 66.4% male), of whom 53.3% had vitamin D deficiency at baseline. In longitudinal models, normal vitamin D levels were associated with lower ASDAS (β per 1 ng/mL increase: −0.010; 95% CI −0.017 to −0.003), BASDAI (β: −0.021; 95% CI −0.036 to −0.006), PGA (β: −0.021; 95% CI −0.039 to −0.003) and BASFI (β: −0.015; 95% CI −0.032 to 0.001). Effect estimates of ASDAS, BASDAI, BASFI, CRP (and their corresponding categorical outcomes) were attenuated in sensitivity analyses but remained directionally consistent. Positive associations with PGA and ASAS-HI were also observed though the estimates were not consistent through all models.Conclusions Normal vitamin D levels were modestly associated with lower disease activity in r-axSpA treated with bDMARDs. Monitoring and optimising vitamin D status may support better disease control.