Gyarados is a piscine, draconic Pokémon with a long serpentine body covered in slightly overlapping scales. It is mostly blue with a yellow underbelly, and it has a row of yellow spots down each side. Its mouth is very large and gaping, bearing four pointed teeth and yellow lips. It has one barbel and a small white fin on each side of its face. The barbels are white on a female and blue on a male. It has small, red eyes, a three-pointed, dark blue crest on its head, and four white, spiky dorsal fins. Gyarados's tail fin is similar in structure to the crest on its head, except with a thin, white fin spread between the points.

The process of evolution causes a change in its brain structure that is believed to cause its violent nature. Gyarados's fangs can crush stones and its scales are harder than steel. Gyarados is feared for its fierce temper and wanton destructive tendencies since ancient times. In some regions, Gyarados is called the "deity of destruction" because of this. It appears in times of conflict and destroys the surrounding area. Its rampages have been known to last a month at a time. These rampages can occur even during a harsh storm. A Gyarados's rage won't stop completely until everything is destroyed. Some believed any place is fated to be destroyed should a Gyarados appear. Gyarados lives in both fresh and salty waters. Both Gyarados and its Mega form have demonstrated in the anime that it can leap huge distances both from the water and from the ground, usually several times its own body length. However, Gyarados is rarely seen in the wild. The move Hyper Beam is often associated with Gyarados due to it being being powerful enough to instantly destroy everything it hits.

When it Mega Evolves, Gyarados becomes bulkier. The fins on its cheeks become longer and gain a yellow tint. Its crest is larger and black, and its barbels also gain considerable length. A large spike extends downward underneath its chin, and it now has a black underside with a red stripe around the edges. The yellow spots along its body are replaced by raised red scales, which propel Mega Gyarados through the water. Two large yellow fins with blue spines appear on its back. Most of the other fins along its back disappear, replaced by two rows of blue spikes. There are now four yellowish, spiky fins near its tail: two on its back and two on its underside.

Due the stress Mega Evolution puts on a Gyarados's body, it becomes even more ferocious than before. All other brain functions cease when Mega Evolution occurs, except for the destructive drive to burn down anything into cinders. Despite its brain having a powerful destructive instinct, it will obey a Trainer it truly trusts. Mega Gyarados can launch at Mach Speed by using its orifices to release water from both sides. With this speed, it has enough power to split large ships in two. Mega Gyarados is tied with Hoopa Unbound as the tallest Dark-type.