Objective:To explore the causal relationship between gut microbiota (GM), immune cell phenotypes, and multiple myeloma (MM) using Mendelian randomization (MR), and to establish the mediating role of immune cells.MethodUsing data from genome-wide association studies, the mediating role of 731 immune cell phenotypes were explored based on two-step MR methods, with GM as exposure, immune cell phenotype as the mediator, and MM as the outcome. The causal associations of GM with MM, immune cell phenotype and MM were first assessed using inverse variance weighted method and multiple methods. After identifying GM and immune cells with significant causal relationships with MM, the causal relationships between current GM and immune cell phenotypes were further explored.ResultsThe results suggested a causal relationship between 9 types of gut microbiota (1 class, 1 order, 2 families, 2 genera, and 3 species) and MM. Additionally, 42 immune cell phenotypes were causally associated with MM. Among these, there were 7 pairs with causal relationships between GM and immune cells. Mediator analysis indicated that IgD on IgD⁺CD24⁺ mediated the risk of Veillonellaceae for MM (mediating effect=5.91%), CD3 on CM CD4⁺(mediating effect=7.28%), and CD3 on secreting Treg (mediating effect=6.70%) both mediated the risk of Ruminococcus1 for MM. Sensitivity analyses further strengthened the robustness of the results.ConclusionThere is a significant causal relationship between GM, immune cell phenotypes, and MM. Modulating the immune system via GM to prevent, monitor, and treat MM is a promising approach.