Barramundi are large fish, with a maximum length of over two meters, although they are more commonly around 1.5 meters. Barramundi can have a mass over 55 kilograms. According to the FAO species identification guide, they have a moderately deep, elongate, and compressed body. Barramundi have pointed snouts and large mouths, with jaws extending past the eyes. Nostrils are close together. The dorsal fin is deeply incised, with separate spiny and soft dorsal fins. The spiny dorsal fin has seven to nine spines, and the soft dorsal fin has ten to eleven soft rays. The anal fin has three spines and seven to eight soft rays. Pelvic and pectoral fins are present. These fish have a distinct caudal peduncle, or tail muscle, with a rounded caudal fin. The lateral line extends onto the caudal fin.

The scales of barramundi are firmly fixed and ctenoid. Adult barramundi are silver with darker, olive or blue-gray backs. In turbid (cloudy) water, coloration is darker. Juveniles are brown (sometimes grayish-brown) with three white stripes on the head and scattered white spots elsewhere. The markings can be dimmed or may disappear at will. The fins do not have markings. The eyes are golden-brown with a red reflective glow.