Description: Adults of S. catenatuss are not large, ranging from 60 to 24 to 30 inches in length. Their color pattern consists of a grey or tan ground color with a row of large, rounded, brown/black blotches or spots down the center of the back and three smaller rows of alternating spots down each side. Solid black melanistic examples are also known, as well as cases where the back blotches join with those on the sides. Young massasaugas are well-patterned, but paler than the adults. They have heat-sensing pits on each side of their smallish heads, their scales are keeled, and their anal scales are single.

Habitat: The species' habitat varies regionally. Habitat in the eastern part of the range includes sphagnum bogs, fens, swamps, marshes, peatlands, wet meadows, and floodplains; also open sanannas, prairies, old fields, and dry woodland; the snakes often occur in wetlands in fall, winter, and spring, in drier adjacent uplands in summer. In Missouri, massasaugas shifted from prairie in spring to upland old fields and deciduous woods in summer, returned to prairie in spring. In the western part of the range, the habitat includes grassy wetland, rocky hillsides, mesquite/scrub plains, thornbrush, oak-grass, dry prairie, desert grassland, and sand dunes of coast and offshore islands. In Texas, this snake often occurs near moist microhabitats or sources of water. In Mexico, massasaugas have been reported from dry shrubby areas that also include wetlands or lush riparian areas. Hibernation occurs in underground burrows (e.g., of mammals or crayfish), crevices, or similar protected sites.

Range: S. catenatus is found in North America from Ontario, Canada and central and western New York throughout the Great Lakes region to the Midwest. It occurs in various habitats ranging from swamps and marshes to grasslands, usually below 1500 m elevation. The type locality given is "... on the prairies of the upper Missouri" (Valley, USA).

Found in these States: IA | IL | IN | MI | MO | NY | OH | PA | WI

Venom: The venom of S. catenatus is a cytotoxic venom, so it destroys tissue. It also contains specialized digestive enzymes that disrupt blood flow and prevent blood clotting. Severe internal bleeding causes the death of the small animals that this snake eats. After envenomation, the rattlesnake is able to withdraw from the dangers of sharp-toothed prey animals until they are subdued and even partially digested by the action of the venom.

S. catenatus is rather shy and avoids humans when it can. Most massasauga snakebites in Ontario have occurred after people deliberately handled or accidentally stepped on one of these animals. Both of these scenarios can be prevented by avoiding hiking through areas of low visibility (in rattlesnake country) when not wearing shoes and long pants and by leaving the snakes alone if encountered. Only two incidents of people dying from massasauga rattlesnake bites in Ontario have been recorded; in both cases, the victims did not receive proper treatment.

Diet: The diet of S. catenatus consists of a variety of small vertebrates, including mammals, birds, bird eggs, lizards, and other snakes, as well as invertebrates such as centipedes and insects. Mammals and reptiles make up the bulk of their diet. Adults feed mainly on rodents (such as voles, white-footed mice, jumping mice, and shrews), while juveniles usually prey on reptiles, more often lizards in western populations and snakes in eastern ones. Frogs also constitute an important part of their diet. S. catenatus feeds on frogs more frequently than any other rattlesnake. In general, however, frogs are not an important part of the diet, although this does seem to be more typical in certain northern and eastern populations.

Reproduction: Eastern Massasauga rattlesnakes mate in the spring and fall. The females hold the babies inside their bodies for about 3.5 months then, rather than laying eggs as some snakes do, they give birth to live young. Females give birth to their 5 to 20 young in abandoned mammal burrows or fallen logs while living in their drier, summer habitats. Young snakes become sexually mature (able to have babies) in their third or fourth year.

Female Eastern Massasauga rattlesnakes nurture and protect their young inside of their bodies while they're developing. The young remain near their mother for a few days after birth and then move away.

Status: The species S. catenatus is classified as least concern on the IUCN Red List of Threatened Species. The population trend is unknown. The eastern massasauga has been listed as a candidate species on the United States Endangered Species Act since 1999.

The eastern massasauga is listed as an endangered species in Illinois, Indiana, Iowa, Minnesota, Missouri (also considered extirpated), New York, Ohio, Pennsylvania, and Wisconsin. Michigan, the only state in which it is not considered endangered, lists it as "special concern". As of 2016, the massasauga is listed as Threatened under the Endangered Species Act.

The eastern massasauga is listed as threatened under both Ontario's Endangered Species Act, 2007, and the federal Species at Risk Act, and is protected under the Fish and Wildlife Conservation Act. It is found only near the eastern shore of Georgian Bay, the Bruce Peninsula, the North Shore of Lake Huron, Wainfleet Bog, and Ojibway Prairie. It is becoming rare in Canada due to persecution and loss of habitat and is designated as "threatened" by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC), as well as the Committee on the Status of Species-at-risk in Ontario (COSSARO).

In Pennsylvania, the species has experienced a rapid decline largely because of habitat loss. Historically, this has been due to human activity and more recently primarily from natural forest succession. By 1988, the snake had disappeared from half of the counties that constituted its historical range. A 2003–2005 survey showed only four locations in two counties with confirmed populations. It is classified as "critically imperiled" to "imperiled" in the commonwealth.

Taxonomy: Three subspecies were recognized for more than a century, although research published in 2011 elevated two subspecies Sistrurus catenatus catenatus and Sistrurus catenatus tergeminus, to full species: the eastern massasauga (Sistrurus catenatus) and the western massasauga (Sistrurus tergeminus). The status of the third subspecies was somewhat unresolved and it is tentatively recognized as the desert massasauga (Sistrurus tergeminus edwardsii) by some, or synonymized with the western massasauga (Sistrurus tergeminus) by others. It is currently thought that eastern massassauga is monotypic; i.e. has no recognized subspecies.