Uedaite-(Ce), ideally Mn2+CeAl2Fe(Si2O7)(SiO4)O(OH), occurs in granite at Shodoshima Island in Seto Inland sea, Kagawa Prefecture, Japan. It is monoclinic, P21/m, a = 8.939(8) Å, b = 5.742(4) Å, c = 10.187(8) Å, β = 115.10(6)°, V = 473.5(6) Å3, Z = 2. The four strongest lines in the powder XRD pattern [d(Å), I/I0, hkl] are (2.92, 100, −302); (3.53, 54, −211); (2.71, 43, 013 120 300) and (2.62, 39, −311). Electron microprobe analysis gave (wt.%); SiO2 29.94, Al2O3 16.02, FeO 16.01, MnO 6.01, MgO 0.07, CaO 2.42, La2O3 3.09, Ce2O3 10.75, Pr2O3 1.83, Nd2O3 6.44, Sm2O3 1.35, Gd2O3 0.54, Y2O3 0.72, ThO2 0.51, and lead to the empirical formula Mn0.51Ca0.26Ce0.39Nd0.23La0.11Pr0.07Sm0.05Y0.04Gd0.02Th0.01Al1.89Fe1.34Mg0.01(Si2O7)(SiO4)O0.85(OH) on the basis of 3 silicon and 1 hydrogen atoms per formula unit. The estimated content of H2O is 1.50 and the total is 97.20. The variation in concentration of Mn in uedaite-(Ce) and associated allanite-(Ce) showed negative linear correlation with that of Ca, suggesting isomorphous substitution of Mn for Ca. The crystal structure of uedaite-(Ce) was refined to wR1 = 0.0337 with single-crystal X-ray diffraction data. The refinement of occupancy parameters for the A and M sites yielded the structural formula A1(Mn0.650Ca0.350) A2(Ce0.914Fe0.086) M1(Al0.873Fe0.127) M2(Al0.988Fe0.012) M3(Fe0.875Al0.125)(Si2O7)(SiO4)O(OH), and confirmed the replacement of Ca with Mn at the A site. Uedaite-(Ce) is a Mn2+-analogue of allanite-(Ce). The calculated density of uedaite-(Ce) is 4.19 g/cm3. Uedaite-(Ce) occurs as short prismatic crystals less than 1 mm length. Crystals are black to dark brown in color. Uedaite-(Ce) is associated with allanite-(Ce), monazite-(Ce), zircon and thorite, and is often altered to bastnäsite-(Ce). The new mineral is biaxial negative and transparent with pleochroism from brown to yellow. However, optical properties are too similar to those of allanite-(Ce), thus distinction by crystal optics is not possible.