IgG4-related disease is a newly recognized systemic disease, and its elucidation is progressing. However, little is known about its sinonasal manifestations. The aim of this study was to assess the olfaction of patients with IgG4-related disease. Twenty-five patients with IgG4-related disease underwent T&T olfactometry to measure olfactory function. We analyzed the clinical features, including serum IgG4 and IgE levels, involved organs, and sinonasal computed tomography scores to explore the etiology of olfactory dysfunction. Thirteen patients with IgG4-related disease were found to have moderate to severe olfactory dysfunction (52%). There were no differences in the clinical features between the olfactory dysfunction group and the normal group. In 7 patients, the inferior turbinate was biopsied to study the correlation between olfaction score and the number of IgG4-positive cells, but no such correlation was found. Six hyposmia patients recovered to a normal state. Five patients recovered after corticosteroid treatment and 1 recovered spontaneously. We found that the prevalence of olfactory dysfunction was high in patients with IgG4-related disease and that it could be reversed. Olfactory dysfunction appears to be a novel important manifestation of IgG4-related disease.