One of the most complex of the many fjords on this coast, it is also one of the largest, 40 kilometres in length and eight kilometres wide at its widest point. To the north of its mouth is the large Resolution Island, whose Five Fingers Peninsula shelters the mouth of the sound from the northwest. along the east coast of the island, Acheron passage connects Dusky Sound with Breaksea Sound, to the north.

Several large islands lie is the sound, notably Anchor Island, Long Island, and Cooper Island. The upper reaches of the sound are steep-sided, and the high precipitation of the region leads to hundreds of waterfalls cascading into the sound during the rainy season. Seals and dolphins are often sighted in the sound’s waters. The Seaforth River is the largest of many small rivers and creeks which flow into the sound.