The living animals were large quadrupeds that were upwards of 4.5 meters in length. Their spines had amphicoelus centra and 3 sacral vertebrae. Their scapulae had large acromion processes. Their forelimbs were much shorter than their hindlimbs, with humeri less than two-thirds the length of their femurs. Their pelvic girdles consisted of a long pubis with a strong symphysis in the middle, a plate-like ischium, a highly recurved ilium, and a deep, imperforate acetabulum. They had relatively long, straight femurs, crurotarsal ankles, and calcaneal tubers that gave them large heels.
Their skulls were relatively small relative to their bodies, on average about 37 centimeters long, 18 centimeters wide, and 15 centimeters high. Their braincases were very firmly fused with the skull roof and palate. They had slender,forked premaxillae that turned up and expanded in the front, creating a shovel-like structure. Desmatosuchus is unique among aetosaurs in that its species are the only known aetosaurs that lacked teeth on their premaxillae. Their premaxillae fit loosely together with their maxillae, indicating flexibility at that joint. Their maxilla contained 10 to 12 teeth. Desmatosuchus also had very thin vomers, which bounded the medial side of the internal nares. These internal nares were relatively large, roughly half the length of the entire palate. Their lower jaw typically carried 5 or 6 teeth, and had a toothless beak on the end. The dentary was about half the length of the lower jaw, with the front portion being toothless and covered by a horny sheath. Behind the dentary was a moderately large mandibular fenestra.
Individuals of Desmatosuchus were heavily armored. The carapace was made up of two rows of median scutes surrounded by two more rows of lateral scutes. The lateral scutes have well-developed spines that point out laterally and dorso-posteriorly. There are typically five rows of spines, and they increase in size anteriorly. The front spine is much larger, around 28 centimeters long, and is recurved. The fourth spine varies in length in each specimen, but remains shorter than the fifth in all of them. Desmatosuchus are the only aetosaurs known to have possessed spines like these.