Magnesiovesuvianite (IMA 2015-104), ideally Ca₁₉Mg(Al,Mg)₁₂Si₁₈O₆₉(OH)₉, a new vesuvianite-group member, was found in an old museum specimen labelled as vesuvianite from the Tuydo combe, near Lojane, Republic of Macedonia. It occurs as radiating aggregates up to 2 cm across consisting of acicular tetragonal crystals (up to 7 mm long and 5–40 μm thick) with distinct fibre-optic effect. Associated minerals are calcite, grossular–andradite and clinochlore. Single crystals of magnesiovesuvianite are transparent, light pink with a silky lustre. Dominant crystal forms are {100}, {110}, {101} and {001}. The Mohs’ hardness is 6. The measured and calculated densities are 3.30(3) and 3.35 g/cm³, respectively. Mineral is optically uniaxial, negative, ω = 1.725(2), ε = 1.721(2). The chemical composition (wt. % electron-microprobe data) is: SiO₂ 36.73, Al₂O₃ 20.21, CaO 36.50, MgO 1.80, MnO 0.18, FeO 0.04, Na₂O 0.01, H₂O 3.10, total 98.37. The empirical formula based on 19 (Ca + Na) apfu is: (Ca_18.99Na_0.01)_{Σ 19.00}(Mg_0.60Al_0.40)_{Σ 1.00}(Al_11.05Mg_0.70Mn_0.07Fe_0.02)_{Σ 11.84}Si_17.84O_68.72(OH)₉. Absorption bands in the IR spectrum are: 393, 412, 443, 492, 576, 606, 713, 802, 906, 968, 1024, 3200, 3450, 3630 cm^–1. The eight strongest lines of the powder X-ray diffraction pattern are (I–d(Å)–hkl): 23–10.96–110, 22–3.46–240, 33–3.038–510, 100–2.740–432, 21–2.583–522, 94–2.365–620, 19–2.192–710, 25–1.6165–672. Magnesiovesuvianite is tetragonal, space group P4/n, unit-cell parameters refined from the powder data are a 15.5026(3), c 11.7858(5), V 2832.4(2) ų, Z = 2. The crystal structure has been refined to R₁ = 0.027 for 3266 unique observed reflections with |F_o| ≥ 4σ_F. The structure refinements provided scattering factors of the Y1A,B sites close to 12 e^– that supports predominant occupancy of these sites by the Mg²⁺ cations, in perfect agreement with the²⁷Al MAS NMR data. Magnesiovesuvianite is a member of the vesuvianite group with Mg²⁺ as a dominant cation at the Y1 site. The name magnesiovesuvianite is given to highlight the species-defining role of Mg.