Deep-sea mud extremely enriched in rare-earth elements and yttrium (together called REY) has been discovered around Minamitorishima Island, in the western North Pacific. The REY-rich mud was previously observed to contain abundant phillipsite and biogenic calcium phosphate. We analyzed the chemical compositions of porewater in a sediment core containing REY-rich mud and observed a decrease of dissolved phosphate (PO43–) from ~1 μM to ~0.5 μM around the REY-rich mud layer. The presence of dissolved nitrate + nitrite throughout the cored depth indicated oxic sediment conditions. In the REY-rich mud, PO43– was presumed to be adsorbed onto the surfaces of minerals such as phillipsite. Our results support that oxic pelagic clay containing phillipsite can be a sink for PO43–. Concentrations of REY in porewater were below the detection limit.