The lionfish head is large, from 1/3 to 1/2 the standard length. Most species have numerous head spines. The dorsal fin has strong, venomous spines. The anal fin also has sharp, venomous spines. The caudal fin is usually rounded or squared, but not forked. This large family contains about 500 species.

The red lionfish has greatly elongated dorsal-fin spines. The membranes of all fins are often spotted. The body is white or cream colored red to reddish-brown vertical stripes. The vertical stripes alternate from wide to very thin (with the thin stripes being more numerous) and sometimes merge along the flank to form a V-shape.

A closely-related species, the devil firefish (Pterois miles), is similar to the lionfish. The devil firefish is found primarily in Indian Ocean and Red Sea (as opposed to the lionfish, which is predominantly a Pacific species); however, its range extends to Sumatra where the two species co-occur. Although it appears very similar to the lionfish, the devil firefish has fewer dorsal- and anal-fin rays. The devil firefish generally has 10 dorsal-fin rays and 6 anal-fin rays; the lionfish usually has 11 dorsal-fin rays and 7 anal-fin rays. See Remarks below. Recent genetic work has revealed that the Atlantic population of lionfish is composed primarily of P. volitans with a small number of P. miles (Hamner et al. 2007).

Widely distributed throughout the western Pacific from southern Japan to Micronesia, Australia and the Philippines. Pterois volitans occurs throughout most of Oceania (including the Marshall Islands, New Caledonia and Fiji) east to French Polynesia. Pterois miles is from the Indian Ocean and Red Sea, although its range extends to Sumatra.