The Obitsu emerges from the densely forested valleys around Mount Motokiyosumi () in the Bōsō Hill Range and empties into Tokyo Bay. It is the second largest river in Chiba Prefecture after the Tone River (). The upper reaches of the Obitsu meander through Kimitsu, the middle of the river runs more directly through Kisarazu, and the lower reaches form and estuary and a triangular delta in Sodegaura, an area known as the Banzu Tidal Flats. The Obitsu then turns briefly west back into Kisarazu to empty into Tokyo Bay. The tidal flats of the Obitsu host numerous species of birds and crustaceans. In 2012 an IUCN Red List endangered species crab, Uca lactea lactea, was discovered at the mouth of the river, and may be the northernmost habitat of the crab in Japan.