The new mineral yellowcatite (IMA2024-030), KNaFe3+2(Se4+O3)2(V5+2O7)·7H2O, was found underground in the School Section #32 mine, Grand County, Utah, USA, where it is a secondary, post-mining phase occurring on montroseite-corvusite-asphaltite-mica-bearing sandstone in association with barnesite, gypsum and mandarinoite. Crystals are thin hexagonal plates, up to ∼0.2 mm in diameter. Crystals are yellow and transparent, with vitreous to pearly lustre and pale-yellow streak. The mineral is brittle with curved fracture and two cleavages: perfect on {001} and good on {100}. The Mohs hardness is ∼2. The measured density is 2.79(2) g·cm–3. Optically, yellowcatite is uniaxial (–) with ω = 1.910(5) and ε = 1.740(5) (white light). The mineral is pleochroic with O yellow and E colourless; O > E. The empirical formula is (K0.65□0.35)Σ1.00(Na0.66Mg0.30)Σ0.96Fe3+2.02Se4+1.99V5+2.01O20H14.02. Yellowcatite is hexagonal, space group P$\bar 6$m2, with cell parameters: a = 5.4966(7), c = 17.2109(16) Å, V = 450.31(13) Å3 and Z = 1. In the crystal structure of yellowcatite (R1 = 5.12% for 281 I > 2σI reflections), Fe3+O6 octahedra, Se4+O3 pyramids and V5+O4 tetrahedra link by corner-sharing to form sheets similar to those in the well-known merwinite structure, but with the apices of the Se4+O3 pyramids in the ‘pinwheels’ pointing in the same direction as the V5+O4 tetrahedra. The unshared vertices of the V5+O4 tetrahedra in adjacent sheets link to one another to form divanadate groups, thereby joining two sheets into a double-sheet slab structural unit. Between adjacent slabs is a layer of unlinked Na(H2O)6 coordinations that are presumed to represent octahedra exhibiting rotational disorder.