Lecoqite-(Y), ideally Na₃Y(CO₃)3^.6H₂O, a new mineral species, was found at the Poudrette quarry, Mont Saint-Hilaire, Quebec, Canada. Lecoqite-(Y) forms radiating, spray-like aggregates, compact, tightly packed masses to 2.5 cm across composed of flexible, extremely thin, capillary crystals up to 2.5 cm long and up to 0.01 mm thick. The empirical formula is: Na_2.94(Y_0.755Dy_0.085Er_0.08Yb_0.035Ho_0.03Gd_0.02Nd_0.015Sm_0.01)∑_1.03(CO₃)_2.91(OH)_0.21(H₂O)_6.06. Lecoqite-(Y) is hexagonal, P₆₃; a = 11.316(4), c = 5.931(2)Å, V = 657.7(4)ų, Z = 2. The crystal structure was studied by Rietveld method from X-ray powder data, based on the model of the isostructural synthetic Ln-free carbonate Na₃Y(CO₃)3^.6H₂O. No mineral is closely related to lecoqite-(Y) in terms of structure. In the structure REE atoms (REE = Y+Ln) are surrounded by six O atoms of CO₃ groups and three H₂O molecules to form a tricapped triangular prism. Na atoms are centered in distorted octahedra [NaO₄(H₂O)₂], which link to form infinite corrugated chains along c. Lecoqite-(Y) is named in the memory of the outstanding French chemist and specialist in the spectroscopic analysis of minerals and synthetic compounds P.E. Lecoq de Boisbaudran (1838-1912), who made a great contribution to the chemistry of the rare earth elements. The Levinson suffix modifier -(Y) is in line with the dominance of yttrium over other rare-earth elements in the mineral. The cotype specimens are deposited in the Fersman Mineralogical Museum of Russian Academy of Sciences, Moscow, and the Canadian Museum of Nature, Ottawa.