The synonym "silver king" refers to the predominant bright silver color along the sides and belly of the tarpon. Dorsally, tarpon usually appear dark blue to greenish-black. However, the color may appear brownish or brassy for individuals inhabiting inland waters. The dorsal and caudal fins have dusky margins and often appear dark.Externally, the almost vertical, silvery sides made up of large scales are the most distinctive feature of the tarpon. The tarpon has a superior mouth with the lower mandible extending far beyond the gape. The fins contain no spines, but are all composed of softrays. The dorsal fin appears high anteriorly and contains 13-15 softrays with the last ray greatly elongated into a heavy filament. The caudal is deeply forked, and the lobes appear equal in length. The anterior portion of the anal fin is deep and triangular. The fin has 22-25 softrays, with the last ray again elongated as in the dorsal fin, but shorter and only present in adults. The tarpon has large pelvic fins, and long pectoral fins containing 13-14 softrays.
Perhaps the most unique internal feature of the tarpon is the modified swim bladder. This swim bladder contains spongy alveolar tissue and has a duct leading to the esophagus that the tarpon may fill directly with air gulped from the surface. This feature allows the tarpon to take oxygen directly from the atmosphere and increases its tolerance of oxygen-poor waters. In fact, studies have shown that tarpon must have access to atmospheric oxygen in order to survive, and that juvenile tarpon are obligatory air-breathers. Adults living in oxygen-rich waters still roll and gulp air, probably as an imitative pattern based on visual perception of other tarpon.