A large, eel-like, aquatic salamander. Greater sirens, like other members of the family Sirenidae, lack hindlimbs and eyelids, have horny beaks on the upper and lower jaws, and are gilled throughout life (Martof 1974). Greater sirens have four toes on the front limbs and three gill slits (Martof 1973; 1974; Petranka 1998). Adults range from 50 - 98 cm total length but most are less than 70 cm (Petranka 1998). The tail is 26 - 40% of the total length (Martof 74). There are ventral and dorsal fins on the tail (Martof 1973). Modal number of costal grooves is 37-38 (Martof 1973; Petranka 1998). Adult males have enlarged jaw (masseter) muscles which make the head appear larger than in females (Petranka 1998). Hatchlings are 16 cm total length (13 cm snout to vent length), and possess a relatively shorter tail than adults and a tail fin which extends from the base of the head to the tip of the tail (Martof 1973).

Adult coloration varies from olive green to light gray above and sometimes there are dark spots on the head, back and sides. The sides are lighter colored than the dorsum and usually have flecks of pale green. The venter is bluish grey and often has pale green flecks. Description from Petranka (1998). Juveniles have light, often yellow, body stripes that fade with age (Martof 1973; Petranka 1998).
 
No subspecies are currently recognized, but geographic variation is poorly understood (Petranka 1998). The recent report (Flores-Villela and Brandon 1992) that greater sirens occur in Texas and Mexico (previously specimens were thought to be S. intermedia) is a large extension to the known range and suggests that detailed surveys across the distribution of greater sirens may uncover interesting patterns.