The coloration is unique, consisting of prominent black "saddles" and large black spots running along the back, on a silvery to bronzy gray background. Adult sharks often have more spots and saddles with lighter centers compared to juveniles. The underside is whitish and plain. The large first dorsal fin is positioned about halfway between the pectoral and pelvic fins; the second is almost as large as the first and much larger than the anal fin. The pectoral fins are wide and triangular. The lower lobe of the caudal fin is well-developed in adults but less than half the length of the upper lobe, which has a strong ventral notch near the tip. The average length of a leopard shark is 1.2–1.5 m (3.9–4.9 ft). Rarely males may grow to 1.5 m (4.9 ft) and females 1.8 m (5.9 ft), and there is a record of an exceptional female that measured 2.1 m (6.9 ft) long. The heaviest known leopard shark weighed 18.4 kg (41 lb)

It is more active at night than during the day, and sometimes lies still on the bottom. In Tomales Bay and elsewhere, the leopard shark follows the tide onto mudflats to forage for food, retreating just fast enough to prevent being stranded or trapped as the water recedes. At Santa Catalina Island, resident sharks spend the day together in the shallows and at night disperse into deeper water, ranging up to 10 km (6.2 mi) away. In captivity, larger sharks have been observed establishing their dominance over smaller individuals via light nips to the pectoral fins. Small leopard sharks fall prey to larger sharks such as the great white shark(Carcharodon carcharias) and the broadnose  sevengill shark(Notorynchus cepedianus).

The diet of the leopard shark consists of small benthic and littoral animals, most significantly crabs, shrimp, bony fish, fish eggs, clams, and the echiurid fat innkeeper worm(Urechis caupo). Young sharks feed mostly on crabs and transition to clam siphons, fish eggs, and innkeeper worms once they reach 70–80 c (2.3–2.6 ft) long. The largest sharks are the ones that consume the most fish. The leopard shark captures prey by expanding its buccal cavity to create a suction force, which is facilitated by its labial cartilages swinging forward to form the mouth into a tube. Simultaneously, the shark protrudes its jaws forward to grip the prey between its teeth. As with other sharks, the teeth of the leopard shark are periodically shed and replaced; it takes 9–12 days for a replacement tooth to move into position. Leopard sharks have been caught with stomachs filled with clam siphons, which the sharks seize before the clams can retract and break off with a levering motion of their bodies. On occasion, the shark tears the entire clam body out of its shell this way. Other sharks examined have had stomachs containing whole innkeeper worms with no bite marks, suggesting that the sharks sucked them out of their burrows.

The leopard shark is aplacental viviparous, with the developing embryos sustained by a yolk sac until birth. Females give birth to 1–37 young annually, with the number of pups increasing with female size. Females drop their pups in beds of eelgrass that provide both shelter and food, give birth on flats in 1 m(3.3 ft) of water, their backs and dorsal fins exposed; the pups stay in even shallower water less than 30 cm (12 in) deep. Newborn leopard sharks measure around 20 cm (7.9 in) long. The maximum lifespan of this species is estimated to be 30 years.

Wary and quick to flee, leopard sharks pose almost no danger to humans. There is a single record from 1955 of a leopard shark harassing a skin diver with a nosebleed, though no injuries resulted. The meat is considered excellent eating and is sold fresh or frozen. The leopard shark is prized by the aquarium trade for its attractive appearance and hardiness. The International Union for Conservation of Nature (IUCN) has assessed the leopard shark as of Least Concern.

Leopard sharks are gregarious and associate with other individuals of similar size and sex. Leopard sharks(Triakis semifasciata) at the Aquarium of the Bay. Author/nugunslinger from Lafayette, USA.

The leopard shark captures prey with a combination of suction and biting. Author/Upsilon Andromedae from Chicago, Illinois, USA.

Young leopard sharks frequent very shallow, coastal waters. A pair of slipper on surface. Leopard shark (Triakis semifasciata) near San Diego. Author/Sung Sook from USA.