Oral lichen planus is a chronic inflammatory disease of the oral mucosa characterised by immune dysregulation, but its pathogenesis remains incompletely understood and no curative treatment has been established. Our previous work showed that patients with oral lichen planus exhibit gut dysbiosis, including reduced microbial diversity and depletion of butyrate-producing bacteria. Because butyrate promotes regulatory T-cell differentiation and supports immune homeostasis, targeting this dysbiosis may represent a microbiota-directed approach for immune modulation in oral lichen planus. This study aims to investigate the effects of inulin supplementation on the gut microbiota and related immune markers in patients with oral lichen planus. This multicentre, double-blind, randomised, placebo-controlled parallel-group trial will enrol 80 patients with oral lichen planus. Participants will be randomly assigned in a 1:1 ratio to receive either inulin (8 g/day) or a maltose placebo for 4 weeks, in addition to standard care. The primary outcome is the change in the relative abundance of prespecified butyrate-producing gut bacteria from baseline to the end of intervention at Week 6. Secondary outcomes include changes in gut microbiota diversity, salivary microbiota composition, faecal short-chain fatty acid concentrations, peripheral blood regulatory T-cell counts, blood test parameters, and clinical symptoms of oral lichen planus. Analyses will follow the intention-to-treat principle, and between-group differences will be assessed using appropriate statistical methods. This trial is designed to evaluate, in a randomised placebo-controlled setting, whether a microbiota-directed intervention can modify gut microbial profiles and related immune markers in patients with oral lichen planus. The findings are expected to clarify whether inulin supplementation can modify gut microbial profiles and related immunological markers in patients with oral lichen planus. Trial registration: UMIN Clinical Trials Registry (UMIN-CTR), UMIN000060840, registered on 1 April 2026 ( https://rctportal.mhlw.go.jp/detail/um?trial_id=UMIN000060840# ).