Zaccariniite, RhNiAs, is a new platinum-group mineral discovered in the Loma Peguera ophiolitic chromitite, Dominican Republic. The mineral forms anhedral grains (1–20 μm in size) and occurs intergrown with garutiite, in association with hexaferrum, Ru-Os-Ir-Fe alloys, Ru-Os-Ir-Fe oxygenated compounds, and chromite. Zaccariniite is opaque and has a metallic luster and a grey streak. The mineral is brittle, with no cleavage. Values of VHN5 (10 indentations on different grains of synthetic RhNiAs) fall between 166 and 286 kg/mm2, with a mean value of 218 kg/mm2, corresponding to a Mohs hardness of approximately 3.5–4. Under plane-polarized reflected light, the mineral is white with brownish to pinkish tints, has moderate to strong bireflectance, a strong white to pinkish brownish white pleochroism, and strong anisotropy, with rotation tints from orange to blue–green; it exhibits no internal reflections. Reflectance values of zaccariniite in air (R1, R2 in %) are: 49.4, 49.4 at 470 nm; 52.4, 53.2 at 546 nm; 54.2, 53.2 at 589 nm; and 56.6, 53.3 at 650 nm. Three electron microprobe analyses of natural zaccariniite gave an average composition of: Rh 41.77, Os 0.51, Ir 0.64, Ru 0.46, Pd 0.34, Ni 23.75, Fe 0.53, As 27.84, S 0.10, for a total of 96.09 wt.%, corresponding to the empirical formula (Rh1.01Os0.01Ir0.01Ru0.01Pd0.01)∑1.05(Ni1.00Fe0.02)∑1.02(As0.92S0.01)∑0.93 based on three atoms. The average result of twenty-eight analyses of synthetic zaccariniite is: Rh 44.57, Ni 24.50, and As 31.82 total 100.88 wt.%, corresponding to Rh1.02Ni0.98As1.00. The mineral is tetragonal, belongs to space group P4/nmm with lattice parameters a = 3.5498(1), c = 6.1573(2) Å, V = 77.59(1) Å3, Z = 2, and crystallizes with the Cu2Sb structure type. The strongest lines in the X-ray powder-diffraction pattern of synthetic zaccariniite [d in Å(I)(hkl)] are: 2.5092(40)(110), 2.3252(100)(111,102), 1.9453(51)(112), 1.7758(80)(103,200), 1.2555(40)(213,220), 1.1044(22)(302,311), 1.0547(23)(312), 0.9730(42)(215). The new Rh-Ni arsenide is named after Federica Zaccarini in recognition of her important contributions to the mineralogy of platinum-group elements.