A hydroxyl-dominant analogue of gadolinite-(Y) (OH-Gad) has been discovered in the Heftetjern granitic pegmatite, southern Norway, in association with late-stage rare-earth-element containing minerals. The empirical formula, based on ten O atoms per formula unit, is (Y 1.285 Ca 0.55 Ce 0.07 La 0.04 Nd 0.01) Σ1.955 (Fe 2+ 0.57 □ 0.43)Be 2.02 Si 1.995 O 8.48 (OH) 1.52. The mineral is monoclinic, space group P2 1/c, a = 4.7514 (10), b = 7.5719 (16), c = 9.9414 (2) Å, β = 90.015 (4)°, V = 357.663 (3) Å 3 and Z = 2. The density calculated using the empirical formula is 3.903 g cm-3. The crystal structure was refined to R = 0.0217 for 776 reflections with I > 2σ(I). OH-Gad is isostructural with gadolinite-(Y) and it is characterized by the predominance of OH-over O 2-at the anionic Ø-site. The refined crystal-chemical formula is: A (Y 1.25 Ca 0.55 Ce 0.2) X (Fe 2+ 0.57 □ 0.43) Z Be 2 T Si ₂O 8 Ø [(OH) 0.86 O 0.59 (OH)∗ 0.55] (Z = 2). The possible orientation and local environment of the hydroxyl group were suggested based on bond-valence sum calculations and geometrical analysis of the crystal structure. The infrared spectrum confirms disordering of H atoms. OH-Gad seems to be a potentially new mineral, the first simultaneously hydroxyl-and iron-dominant member of the gadolinite subgroup. It is an OH-analogue of gadolinite-(Y) and an Fe 2+-analogue of hingganite-(Y).