Eu³⁺-doped gadolinium oxyhydroxide Gd_1-xEu_xOOH crystals were synthesized by the flux method. The X-ray diffraction data for the crystals were well refined assuming a monoclinic structure with the P2₁/m space group. Gd_1-xEu_xOOH (x ≤ 0.2) crystals showed strong red emission, and the highest fluorescence quantum yield (Φ_f) was 0.27, obtained for x = 0.10. Φ_f decreased rapidly as the Eu³⁺ content x increased above 0.2, owing to concentration quenching. Analysis with a percolation model indicated three-dimensional energy transfer between the Eu³⁺ ions.