Red-bellied piranhas have an average length of 6 to 8 inches (15 to 20 centimeters) but can grow to 12 inches (30 centimeters). The belly, the throat, the pectoral (PECKter-uhl) fins, or the front pair, the pelvic fins, or the rear pair, and the anal fins are bright red. The sides are often pale brown to slightly olive, and the back is bluish gray to brownish. The powerful jaws have sharp, triangular, interlocking teeth.

Red-bellied piranhas look for food mainly at dusk and dawn. After a courtship display involving swimming in circles, the female deposits layers of eggs on plants in the water, and the male fertilizes them. The male guards the egg masses and fans them with his fins until the eggs hatch in nine or ten days..