Mendeleevite-(Nd), (Cs,□)₆(□,Cs)₆(□,K)₆(REE,Ca)₃₀(Si₇₀O₁₇₅)(OH,H₂O,F)₃₅ is a new mineral from the Darai-Pioz alkaline massif, Tajikistan. Mendeleevite-(Nd) was found in a pectolite aggregate in silexites (quartz-rich rocks) which consist of fine to medium pectolite grains, quartz, aegirine and fluorite, with minor khvorovite, mendeleevite-(Ce), sokolovaite, hyalotekite, orlovite, kirchhoffite, pekovite, neptunite, zeravshanite, senkevichite, nordite-(e), alamosite, pyrochlore-group minerals and baratovite. Mendeleevite-(Nd) forms colourless cubic crystals 10-40 μm in size; it has a vitreous lustre and a Mohs hardness of 5-5.5; D_meas. = 3.20(2) g/cm³, D_calc. = 3.155 g/cm³. Mendeleevite-(Nd) is optically isotropic, with the refractive index n = 1.582(2). Mendeleevite-(Nd) is cubic, space group Pm3, a = 21.9106(4) Å; Z = 2. The six strongest reflections in the powder X-ray diffraction pattern are [d (Å), I (%), (h k l)] are: 11.01, 100, (0 0 2); 15.63, 55, (0 1 1); 3.47, 42, (2 0 6); 3.099, 42, (3 4 5); 2.192, 42, (0 0 10); 1.819, 41, (3 6 10). Chemical analysis by electron microprobe gave SiO₂ 42.30, Ce₂O₃ 10.12, La₂O₃ 3.60, Nd₂O₃ 16.19, Pr₂O₃ 2.79, Sm₂O₃ 4.19, Gd₂O₃ 1.69, Eu₂O₃ 0.47, SrO 2.99, CaO 2.20, Cs₂O 8.50, K₂O 0.85, H₂O 3.85, F 1.25, -O = F₂ -0.53, sum 100.46 wt.%, with H₂O calculated by analogy with mendeleevite-(Ce). The empirical formula based on 210 (O + F) apfu, with F + OH + H₂O = 35 pfu, is Cs₆(□_4.20K_1.80)_Σ6{[(Nd_9.57Ce_6.13Sm_2.39La_2.20Pr_1.68Gd_0.93Eu_0.27)_Σ23.17(Ca_3.90Sr_2.87)_Σ6.77]_Σ29.94□_0.06}_Σf30(Si₇₀O₁₇₅)(OH, H₂O,F)₃₅ and Cs₆(REE₂₃Ca₇)(Si₇₀O₁₇₅)(OH,F)₁₉(H₂O)₁₆, respectively. The compatibility index (from measured density) = -0.039 (excellent). Mendeleevite-(Nd) is a Nd analogue of mendeleevite-(Ce), (Cs,□)₆(□,Cs)₆(□,K)₆(REE,Ca,□)₃₀(Si₇₀O₁₇₅)(H₂O,OH,F,□)₃₅. Both minerals are named after Dmitri Mendeleev (1834-1907), the great Russian chemist, author of the periodic table of chemical elements, who has had a significant impact on the development of natural sciences and industry, both in Russia and around the world.