Hydroxynatropyrochlore, (Na,Ca,Ce)₂Nb₂O₆(OH), is a new Na-Nb-OH-dominant member of the pyrochlore supergroup from the Kovdor phoscorite-carbonatite pipe, Kola Peninsula, Russia. It is cubic, Fd3m, a = 10.3211(3) Å, V = 1099.46(8) ų and Z = 8 (from powder diffraction data) or a = 10.3276(5) Å, V = 1101.5(2) ų and Z = 8 (from single-crystal diffraction data). Hydroxynatropyrochlore is a characteristic accessory mineral of the low-carbonate phoscorite in the contact zone of the phoscorite-carbonatite pipe with host foidolite as well as in the carbonate-rich phoscorite and carbonatite of the pipe axial zone. It usually forms zonal cubic or cubooctahedral crystals (up to 0.5 mm in diameter) with irregularly shaped relics of amorphous U-Ta-rich hydroxykenopyrochlore inside. Characteristic associated minerals include rock-forming calcite, dolomite, forsterite, hydroxylapatite, magnetite and phlogopite, accessory baddeleyite, baryte, barytocalcite, chalcopyrite, chamosite-clinochlore, galena, gladiusite, juonniite, ilmenite, magnesite, pyrite, pyrrhotite, quintinite, spinel, strontianite, valleriite and zirconolite. Hydroxynatropyrochlore is pale brown, with an adamantine to greasy lustre and a white streak. The cleavage is average on {111} and the fracture is conchoidal. Mohs hardness is ~5. In transmitted light, the mineral is light brown, isotropic and n = 2.10(5) (λ = 589 nm). The calculated and measured densities are 4.77 and 4.60(5) g cm^-3, respectively. The mean chemical composition determined by electron microprobe is: F 0.05, Na₂O 7.97, CaO 10.38, TiO₂ 4.71, FeO 0.42, Nb₂O₅ 56.44, Ce₂O₃ 3.56, Ta₂O₅ 4.73, ThO₂ 5.73, UO₂ 3.66, total 97.65 wt.%. The empirical formula calculated on the basis of Nb + Ta + Ti = 2 apfu is (Na_1.02Ca_0.73Ce_0.09Th_0.09 U_0.05Fe_0.02²⁺)Σ2.00(Nb_1.68Ti_0.23Ta_0.09)Σ2.00O_6.03(OH_1.04F_0.01)Σ1.05. The simplified formula is (Na,Ca,Ce)₂Nb₂O₆(OH). The mineral dissolves slowly in hot HCl. The strongest X-ray powder-diffraction lines [listed as (d in Å)(I)(hkl)] are as follows: 5.96(47)(111), 3.110(30)(311), 2.580(100)(222), 2.368(19)(400), 1.9875(6)(333), 1.8257(25)(440) and 1.5561(14)(622). The crystal structure of hydroxynatropyrochlore was refined to R₁ = 0.026 on the basis of 80 unique observed reflections. The mineral belongs to the pyrochlore structure type A₂B₂O₆Y₁ with octahedral framework of corner-sharing BO₆ octahedra with A cations and OH groups in the interstices. The Raman spectrum of hydroxynatropyrochlore contains characteristic bands of the lattice, BO₆, B-O and O-H vibrations and no characteristic bands of the H₂O vibrations. Within the Kovdor phoscorite-carbonatite pipe, hydroxynatropyrochlore is the latest hydrothermal mineral of the pyrochlore supergroup, which forms external rims around grains of earlier U-rich hydroxykenopyrochlore and separated crystals in voids of dolomite carbonatite veins. The mineral is named in accordance with the pyrochlore supergroup nomenclature.