Corundum-rich garnetite occurs as an isolated lens in a garnet peridotite body in the Donghai area of the Sulu ultrahigh-pressure (UHP) terrane. This rock consists of garnet and corundum, along with minor crack-related zoisite, pargasite, Mg-staurolite, Mg-chloritoid, sapphirine and chlorite. Pyropic garnet (Prp54-63Grs26-36Alm10-12) exhibits a sinusoidal REE pattern, positive Ta, Pb, and negative Nb, Ti anomalies due to metasomatism. Reddish corundum contains 1.1-1.7 wt% Cr2O3, and shows three oriented sets of exsolved rutile needles. Both garnet and corundum contain inclusions of apatite, Mg-allanite (MgO > 4 wt%), and Ni-Fe sulfides formed as trapped Ni-Fe-S melt. The protolith of the corundum-rich garnetite could have been spinel websterite formed in the upper mantle. Both the websterite and the host garnet peridotite were subjected to subduction-zone UHP metamorphism at ≥800 °C and > 4 GPa. Crack-related hydrous phases were formed by fluid infiltration during exhumation.