Sharks, which are uniformly considered carnivores, have guts optimized for digesting a high-protein diet. Omnivores, on the other hand, also digest plant material, and thus, face the difficulty of digesting carbohydrates and foods sheathed in rigid cell walls. The bonnethead shark (Sphyrna tiburo) is known to consume copious amounts of seagrass (up to 62% of gut content mass in juveniles of some populations), yet maintains a gut that morphologically reflects its carnivorous ancestry. We investigated the digestive function of S. tiburo in order to determine whether they can digest and assimilate nutrients from seagrass (Thalassia testudinum). S. tiburo were held in captivity and fed a 90% seagrass diet equaling 5% of their body weight daily for three weeks. By growing the seagrass in a separate tank containing enriched 13CO2, the seagrass tissues became labeled with 13C. Weekly blood draws from the sharks consuming the labeled seagrass show (via stable isotope analysis) that they are assimilating carbon from the labeled seagrass. Digestibility analyses show that 58.6 ± 2.9% of the total organic matter in the 90% seagrass diet is digested by S. tiburo. A spike in β-glucosidase activity (a cellulose-degrading enzyme) was recorded in S. tiburo hindguts. Hence, whether or not the ingestion of seagrass is incidental, these results provide explicit evidence that bonnethead sharks, animals previously thought to be solely carnivorous, can benefit from the digestion of seagrass, which leads one to re-evaluate the ecological role of S. tiburo in its coastal habitats.