The new zeolite martinandresite, ideally Ba2(Al4Si12O32)·10H2O, was discovered in the armenite locality of Wasenalp near the Isenwegg peak, Ganter valley, Simplon region, Switzerland. The associated minerals are armenite, quartz, dickite, and chlorite. Martinandresite forms tan-coloured blocky crystals up to 8 × 5 × 3.5 mm, their aggregates up to 6 cm across, as well as cruciform twins up to 3.5 mm. The major form is {010}; the subordinate forms are {100} and {001}. Indistinct cleavage is observed, presumably on (010) and in a direction across (010). The Mohs’ hardness is 4½. Density measured by flotation in heavy liquids is 2.482(5) g/cm3. Density calculated using the empirical formula is equal to 2.495 g/cm3. Martinandresite is optically biaxial, negative, α = 1.500(2), β = 1.512(2), γ = 1.515(2) (λ = 589 nm). 2V (meas.) = 55(10)°. The IR spectrum is given. The chemical composition of martinandresite is (wt%; electron microprobe, H2O determined by the modified Penfield method): Na2O 0.37, K2O 0.12, BaO 21.55, Al2O3 15.03, SiO2 49.86, H2O 12.57, total 99.50. The empirical formula based on 16 atoms Si + Al pfu is Na0.17K0.04Ba2.00(Al4.19Si11.81O32)H19.85O9.93. The crystal structure was determined using single-crystal X-ray diffraction data. The new mineral is orthorhombic, space group Pmmn, with a = 9.4640(5), b = 14.2288(6), c = 6.9940(4) Å, V = 941.82(8) Å3 and Z = 1. The crystal structure of martinandresite is unique and is based on the Al–Si–O tetrahedral framework containing four-, six- and eight-membered rings of tetrahedra. Si and Al are disordered between the two independent tetrahedral sites. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 6.98 (74) (001), 6.26 (83) (011), 5.61 (100) (101), 3.933 (60) (220, 031), 3.191 (50) (112), 3.170 (62) (041), 3.005 (79) (231, 141). Martinandresite is named after Martin Andres (b. 1965), the discoverer of the armenite locality of Wasenalp.