The Utila Iguana is a medium sized Spiny-tailed Iguana with males reaching up to 800 mm total length with an SVL of 310 mm and weight up to 900 g. Females are about 30% smaller. The ratio of tail length to SVL is 1.47 – 1.74. Males have a well-developed dewlap and impressive dorsal crest.

Juveniles are exclusively blackish brown with some dark brown dorsal striping (unlike most other species of ctenosaur, where juveniles are partly or entirely green). Adults are predominantly grey-brown to turquoise blue with some dark shading. The tail has whorls of enlarged spiky scales in dark and light bands. The dorsal crests of males consist of dark and light spines arranged in alternating rows of two or three spines of each color.