This bandicoot shows some sexual dimorphism, with females being sightly smaller than males. The average male length is 330 mm, with a tail of 120 mm. Females are about 30 mm shorter, with a 10 mm shorter tail. Males weigh an average of 0.9 kg, females 0.7. The fur of this marsupial is coarse and colored a dark greyish to yellowish brown, with the undersides a creamy-white. It has short, round ears.
Reproduction is closely linked to local rainfall pattern, and many brown bandicoots breed all year around. A litter of up to 5 young is born after an 11-day gestation and is weaned at 2 months.
While some authorities list as many as five subspecies (I. o. fusciventer, I. o. obesulus, I. o. peninsulae, I. o. affinus, I. o. nauticus), the most recent edition of Mammal Species of the World only lists I. o. nauticus as a valid subspecies, aside from the nominate; the others are given synonym status.