The new ferric sulphate mineral, mikasaite, the Fe analogue of millosevichite was found in Ikushunbe tsu, Mikasa city, Hokkaido, Japan. Mikasaite occurs as a sublimate around fractures from which coal gas escapes. The white-light brown coloured mineral shows aggregates of fine porous crystals. The SEM micrographs indicate that the aggregates are hollow spherical crystals of 100 µm average diameter and 1–5 µm thickness. Microprobe analysis, SO₃ by wet analysis and H₂O by moisture evolution analyser gives Fe₂O₃ 24.3, Al₂O₃ 4.3, Mn₂O₃ 0.5, SO₃ 46.8, H₂O⁽⁻⁾ 23.0, total = 98.9 wt.%. The specimen adsorbed a large amount of H₂O on its surface because of its strong deliquescence. As H₂O is not essential to the mikasaite structure, the empirical formula on the basis of 3SO₄ is (Fe_1.56Al_0.44Mn_0.03)_Σ2.03(SO₄)_3.00, ideally Fe₂(SO₄)₃. The strongest 10 lines in the X-ray powder diffraction patterns, indexed on a hexagonal unit cell are (d,I/I_o,hkl): 3.56, 100, 113; 5.99, 28, 012; 4.35, 23, 104; 2.97, 20, 024; 2.72, 20, 116; 2.64, 11, 211; 2.35, 7, 300; 2.24, 6, 303; 1.78, 6, 226; 3.68, 5, 006. It has space group R with hexagonal lattice parameters of a= 8.14(1) and c = 21.99(8) Å.