Abstract δ-AlOOH has recently been reported to be the least compressible hydrous phase known with a compressibility comparable to corundum and stishovite. It is predicted to be stable at the lower mantle conditions of the earth. By means of first principles methods, we have studied phase stability, electronic structure, elastic and optical properties of δ-AlOOH. Comparison between the elastic constants of δ-AlOOH and α-Al 2O 3 (corundum) shows that δ-AlOOH is as hard as corundum even though it contains 15% amount of water. In addition, δ-AlOOH has a calculated refractive index n similar to that of corundum.