What is the largest living rodent species?

The capybara is the largest living rodent. Its scientific name is Hydrochoerus hydrochaeris, and adults have a heavy, barrel-shaped body that can weigh far more than familiar rodents such as beavers, marmots, and nutria.

Capybaras are semiaquatic herbivores native to much of South America. They live near rivers, ponds, marshes, and other water sources, eating grasses and aquatic plants. Slightly webbed feet and eyes, ears, and nostrils positioned high on the head help them move through water and escape predators.

Despite their size, capybaras are closely related to guinea pigs and rock cavies. They are not oversized beavers: capybaras have no broad, flattened tail, while beavers use their tails for swimming, balance, and communication. Nutria are also rodents, but are substantially smaller.

The largest recorded wild capybaras have exceeded 90 kilograms, although typical adults are lighter. The extinct Josephoartigasia monesi was much larger and may have been the biggest rodent ever known, but it does not change the answer because the question asks about living species.