Momoiite, (Mn²⁺,Ca)₃(V³⁺,Al)₂Si₃O₁₂, an Mn²⁺- and V³⁺-rich analog of silicate garnet, was found in three localities: the Kurase (type locality), Hokkejino, and Fujii mines that are respectively located in Ehime, Kyoto, and Fukui Prefectures, Japan. Momoiite is greenish-yellow to emerald-green in color, and it occurs in the form of an aggregate having a size of up to 1 mm in a vein having a thickness of several millimeters with calcite and tephroite. Momoiite is optically isotropic, with n = 1.86(1). Its calculated density is 4.01 g/cm³, and its hardness on the Mohs scale is 6.5. The representative empirical formula of momoiite from the Kurase mine is (Mn_1.80Ca_1.12Mg_0.04)_Σ2.96(V_1.51Al_0.52)_Σ2.03Si_3.00O₁₂ on the basis of O = 12 per formula unit. The mineral is cubic with a space group of Ia3d, a = 11.9242(7) Å, V = 1695.5(3) ų, and Z =8. The strongest seven lines in the X-ray powder diffraction pattern [d in Å (I/I₀) (hkl)] are 2.982 (78) (400), 2.668 (100) (420), 2.435 (42) (510), 2.178 (20) (521), 1.935 (21) (611), 1.654 (20) (640), and 1.594 (30) (642). A momoiite solid solution substantially exists at relatively low pressure; however, extremely high momoiite content may not be formed in the pressure conditions of ore deposits in the Kurase, Hokkejino, and Fujii mines even if V and Mn are highly concentrated in the ore.