Based on a study of samples found in the Khibiny (Mt. Rasvumchorr: the holotype) and Lovozero (Mts Alluaiv and Vavnbed) alkaline complexes on the Kola Peninsula, Russia, tinnunculite was approved by the IMA Commission on New Minerals, Nomenclature, and Classification as a valid mineral species (IMA no. 2015-02la) and, taking into account a revisory examination of the original material from burnt dumps of coal mines in the southern Urals, it was redefined as crystalline uric acid dihydrate (UAD), C₅H₄N₄O₃ · 2H₂O. Tinnunculite is poultry manure mineralized in biogeochemical systems, which could be defined as “guano microdeposits.” The mineral occurs as prismatic or tabular crystals up to 0.01 × 0.1 × 0.2 mm in size and clusters of them, as well as crystalline or microglobular crusts. Tinnunculite is transparent or translucent, colorless, white, yellowish, reddish or pale lilac. Crystals show vitreous luster. The mineral is soft and brittle, with a distinct (010) cleavage. D_calc = 1.68 g/cm³ (holotype). Tinnunculite is optically biaxial (–), α = 1.503(3), β = 1.712(3), γ = 1.74(1), 2V_obs = 40(10)°. The IR spectrum is given. The chemical composition of the holotype sample (electron microprobe data, content of H is calculated by UAD stoichiometry) is as follows, wt %: 37.5 О, 28.4 С, 27.0 N, 3.8 H_calc, total 96.7. The empirical formula calculated on the basis of (C + N+ O) = 14 apfu is: C_4.99H₈N_4.07O_4.94. Tinnunculite is monoclinic, space group (by analogy with synthetic UAD) P2₁/c. The unit cell parameters of the holotype sample (single crystal XRD data) are a = 7.37(4), b = 6.326(16), c = 17.59(4) Å, β = 90(1)°, V = 820(5) ų, Z = 4. The strongest reflections in the XRD pattern (d, Å–I[hkl]) are (Formula presented.).