The marsh rabbit (Sylvilagus palustris) is a small cottontail rabbit found in marshes and swamps of coastal regions of the Eastern and Southern United States, from Virginia to the lower Florida Keys. Being a small brown rabbit, it is similar in appearance to the widespread eastern cottontail (Sylvilagus floridanus), but is characterized by its smaller ears, legs, and tail, as well as its grayish-brown underparts.

Marsh rabbits are common throughout their range, except for in the Everglades, where the invasive Burmese python has reduced their numbers, and the lower Florida Keys, where the subspecies known as the Lower Keys marsh rabbit (Sylvilagus palustris hefneri) is threatened by rising sea levels and human development of the land. They prefer to live in marshes, swamps, and alongside coastal waters and rivers, as they are excellent swimmers. The marsh rabbit is often compared to the aquatic swamp rabbit (Sylvilagus aquaticus), which is found along the Gulf coast and further inland in the southeastern United States, though the swamp rabbit has larger ears, tail and legs, as well as a distinctly white underside of its tail.

Like other rabbits, the marsh rabbit is a herbivore, and will eat the leaves and bulbs of various aquatic plants, such as cattails and marsh pennywort. It will also eat crops, such as carrot, sweet potato, and sugar cane, which has caused it to be labeled as a pest. The marsh rabbit is nocturnal and will dig out shelters in the ground with its long toenails to hide and rest in during the daytime, and though it does not create burrows like some other rabbits, it will take advantage of those left behind by other animals. The rabbit's long nails also help it to swim. Some predators of the marsh rabbit include birds of prey, such as owls and the northern harrier, and land animals like bobcats and snakes. Rabbit ticks and other parasites are often found on and in the marsh rabbit, and cases of tularemia and mange are known from it as well.

Humans have some history with marsh rabbits, with the species' diet of aquatic and cultivated plants making it considered as a pest, especially in Florida where sugar cane is grown. Some states regulate marsh rabbits as game animals for hunting, where patches of dried grass may be burned to flush them out. As food, the marsh rabbit may be stewed, or it may be marinated, floured, and fried. The term "marsh rabbit" is used in some restaurants in the eastern United States as a euphemism for muskrat meat. Due to how common it is across most of its range, the marsh rabbit is classified as a least-concern species by the International Union for Conservation of Nature.

The marsh rabbit was first described by the American naturalist John Bachman in 1837, who had discovered the species in South Carolina 15 years prior and gave the specimen he recovered to the Charleston Museum. As the specimen had gone undescribed by all other naturalists during that period, Bachman used his knowledge of the rabbits he had examined since then to describe the new species Lepus palustris, the "swamp hare". Bachman noted it as being different from the snowshoe hare (Lepus americanus) in that it would readily swim through bodies of water rather than going around them. Its type locality was restricted to be "eastern South Carolina" in 1901. In 1904, it was placed in the genus Limnolagus by American mammalogist Marcus Ward Lyon Jr.; the name was previously used as the subgenus of both the marsh rabbit and swamp rabbit (Sylvilagus aquaticus) in an 1899 work by the Swiss physician Charles Immanuel Forsyth Major. Edward William Nelson, an American naturalist, clarified several years later in 1909 that the species belonged to Sylvilagus and grouped it alongside the larger swamp rabbit, which is native to the southeastern United States and Gulf Coast as far east as South Carolina and is not synoymous with the marsh rabbit.

The marsh hare's scientific name, Sylvilagus palustris, is derived from Latin. The genus name sylvilagus is a combination of silva ('forest') and lagos ('hare'), meaning "hare of the woods". The species name palustris means "marsh loving".

Fossils of the marsh rabbit are known from Florida, South Carolina, and Georgia. The oldest of these fossils may be from the Rancholabrean age of North America, about 774,000 years ago, but most are found in deposits that are dated to the late Pleistocene age. Much of the fossil evidence of marsh rabbits comes from deposits in the coastal plains of Melbourne, Florida, and is largely between 125,000 and 75,000 years old. An increase in rain throughout this region in the period 6,500 to 5,000 years ago is considered the main cause of the marsh rabbit's spread throughout its current distribution.

In the subgenus Tapeti in the genus Sylvilagus, marsh rabbits and swamp rabbits share a chromosomal karyotype derived from a common ancestor and have a diploid number of 2n=38. Molecular data analysis from sequencing the mitochondrial 12S ribosomal RNA gene, which has been considered particularly useful in determining genetic relationships between species, confirms that S. palustris and S. aquaticus are sister taxa.