The Koito River originates from a deep valley in forests around Mount Kiyosumi in the Bōsō Hill Range. The river cuts through the Kazusa Hill Range, a smaller part of the Bōsō Hill Range, and in its upper part, meanders northwest. The Koito creates picturesque ravines throughout the Bōsō Hill Range, and passes by Mount Karasuba and Mount Kiyosumi. The middle and lower parts of the Koito feature fluvial terraces. Extensive land reclamation was carried out near the mouth of the river, and this land was used to build the Nippon Steel Kimitsu Works. The Kimitsu Works sits on the north bank of the Koito River where it empties in Tokyo Bay.