Vitamin D deficiency has previously been linked to higher rates of exacerbation and reduced lung function in asthmatics. Previous randomised controlled trials (RCT) investigating the effect of vitamin D supplementation have mainly focussed on children with asthma. Trials involving adults have typically used bolus dosing regimes and the main outcomes have been patient focussed without investigating underlying inflammation. The present study aimed to conduct a 12-week placebo-controlled RCT administering a daily 5000 IU (125 µg) vitamin D3 supplement to adults with mild to moderate asthma. A total of 32 participants were randomised to receive either the 5000 IU vitamin D3 supplement or an identical matching placebo. The primary outcome of the study was lung function measured by ratio of FEV₁:FVC (effect size 2.5) with secondary outcomes including asthma symptoms and inflammatory biomarkers. There was a small but statistically significant higher increase in the mean (± SD) ratio of FEV₁: FVC from baseline to post-intervention in the vitamin D group (+ 0.05 ± 0.06) compared to the placebo group (+ 0.006 ± 0.04, p = 0.04). There was no effect of the intervention on asthma control test scores, or the inflammatory biomarkers measured. There was a moderate, significant association between baseline plasma 25(OH)D concentration and baseline plasma IL-10 (r = 0.527, p = 0.005) and TNF-α (r = -0.498. p = 0.008) concentrations. A daily vitamin D3 supplement led to slightly improved lung function in adult asthmatics and may be a useful adjunct to existing asthma control strategies, particularly for individuals with suboptimal vitamin D status.