Background/Question/Methods The American horseshoe crab, Limulus polyphemus, is an ecologically-, economically-, and biomedically-important species that lives in coastal and offshore environments along the Atlantic and Gulf coasts of the U.S.A and the Yucatan Peninsula of Mexico. Every spring and summer, as water temperatures begin to rise, horseshoe crabs come ashore to spawn, with females burying their eggs in the sediment at high tide. Traditionally, horseshoe crabs were thought to primarily use sandy beaches for spawning, with other habitats considered suboptimal for embryonic development. Recent observations from South Carolina, however, indicate that horseshoe crabs also spawn in salt marshes. The estuaries of South Carolina comprise extensive (> 300,000 acres) salt marshes which may represent a previously undervalued and overlooked spawning habitat for horseshoe crabs. We investigated the importance of salt marshes as horseshoe crab spawning habitat using multiple approaches. First, we conducted spawning surveys in beach and salt marsh habitats. Next, we dug for eggs and embryos at selected points on beaches and in salt marshes to compare the relative likelihood of spawning in each habitat. Finally, we staged eggs, embryos, and trilobites found during our surveys to compare the developmental progress of eggs laid in each habitat type. Results/Conclusions We observed horseshoe crabs spawning in both beach and salt marsh habitats during our spawning surveys. Furthermore, our surveys of selected points found eggs, embryos, and trilobites in both habitat types, but trilobites were more likely to be found at beach sites than at marsh sites. This suggests that embryonic development can occur in the salt marshes, albeit perhaps more slowly than at beach sites. Our results indicate that horseshoe crabs likely spawn extensively in the salt marshes of South Carolina and that these environments are a potentially valuable source of recruitment for this species. This underappreciated source of recruitment for horseshoe crabs derived from salt marsh habitat may need to be incorporated into population assessments for this species, which often use data from traditional spawning surveys conducted in sandy beach habitat. Studies into the use and impact of non-sandy beaches as spawning habitat in other parts of the geographic range of L. polyphemus, particularly where extensive salt marshes are present, are warranted. The use of salt marshes and other alternative spawning habitats may increase as warming, development, and other anthropogenic effects reduce the availability of traditional horseshoe crab spawning habitats.