Cesiokenopyrochlore is a new mineral belonging to the pyrochlore group. It was discovered in a specimen of granitic pegmatite collected at Tetezantsio, Betafo region, Madagascar. The mineral forms rough equant crystals up to 0.05 mm in size intergrown with béhierite and rynersonite. Cesiokenopyrochlore is light-brown, translucent, with resinous luster. D _calc. ¼ 5.984 g/cm ³. In reflected light it is light gray, isotropic, with strong light-brown internal reflections. The crystal structure was refined to R ₁ ¼ 0.0212. The mineral is cubic, Fd ^-3m, a ¼ 10.444(1) A , V ¼ 1139.5(2) A ³, and Z ¼ 8. The strongest lines of the powder X-ray diffraction pattern [d, A , (I, %) (hkl)] are: 6.03 (37) (111), 3.70 (9) (220), 3.15 (100) (311), 3.02 (36) (222), 2.012 (17) (511, 333), 1.848 (19) (440), 1.576 (11) (622). The chemical composition is (wt.%; electron microprobe): Cs ₂O 22.66, Na ₂O 1.74, CaO 0.64, Nb ₂O ₅ 20.87, Ta ₂O ₅ 21.27, WO ₃ 30.67, H ₂O (calc) 0.12, total 97.97. The empirical formula of the holotype specimen calculated on the basis of (NbþTaþW) ¼ 2 apfu and (OþOH) ¼ 6 apfu and written according to the pyrochlore-supergroup nomenclature is Na _0.29Ca _0.06(Nb _0.81W _0.69Ta _0.50) _R2[O _5.93(OH) _0.07] _R6Cs _0.83. The simplified formula of the holotype specimen is A ₂(Nb,W,Ta) _R2O ₆Cs. Cesiokenopyrochlore is the first natural niobate to adopt the inverse pyrochlore structure.