The management of psoriasis remains a challenge for dermatologist and patient. This study aimed to determine whether vitamin D supplementation improves psoriasis compared to placebo.In a randomized, doubled-blind, placebo-controlled trial, 101 participants ≥18 years with psoriasis were grouped by severity and allocated to 100,000 International Units (IU) vitamin D/month for 12 months (200,000 IU at baseline; n = 67) or an identical placebo (n = 34). Psoriasis Area and Severity Index (PASI) and serum 25(OH)D concentrations were assessed at 3-monthly intervals. The primary outcome was the difference in PASI between groups over time. The relationship between 25(OH)D and PASI across the sample was also considered in a post hoc analysis.PASI did not differ between groups at any time (group F(1, 104) = 0.48, p = .49; group*time F(4, 384) = 0.26, p = .90). However, 25(OH)D increased in both groups, rendering these findings inconclusive. A significant inverse relationship existed between PASI and 25(OH)D, with elevation of 25(OH)D by up to 125 nmol/L associated with mild decreases in PASI (estimated range of decrease 0-2.6; p = .002).A direct benefit of vitamin D supplementation for psoriasis could not be determined. However, these findings suggest a relationship between 25(OH)D and psoriasis severity, at least in some subgroups. Australian New Zealand Clinical Trials Registry #12611000648921.