We investigated rhizoremediation using Phragmites australis (common reed) for fresh sediment contaminated with 4-n-butylphenol (4-n-BP). We collected P. australis plant, rhizosphere sediment, and unvegetated sediment samples from a natural pond and conducted degradation experiments using sediments spiked with 4-n-BP. 4-n-BP was rapidly removed from the rhizosphere sediment with P. australis plants, whereas 4-n-BP persisted in the unvegetated sediment without plants and in the autoclaved rhizosphere sediment with P. australis plants, suggesting that the accelerated 4-n-BP removal resulted largely from the biodegradation by rhizosphere bacteria rather than by adsorption and uptake by the plant. Four bacterial strains, Pseudomonas spp. TO-1, TO-2, TO-3, and TO-4, capable of utilizing 4-n-BP as a sole carbon source via a meta-cleavage pathway, were isolated from the rhizosphere sediment. These strains presumably contributed to the accelerated 4-n-BP removal from the rhizosphere sediment. Oxygen and organic compounds from the roots elevated both cell numbers and 4-n-BP-degrading activity of strain TO-4 (the only strain tested) in the rhizosphere sediment, leading to rapid 4-n-BP removal. The results suggest that rhizoremediation using P. australis can be applicable to remove 4-n-BP from the contaminated sediments.