Wisconsin has 2 venomous snakes named by official sources: Eastern Massasauga and Timber Rattlesnake. Each one is labelled venomous by University of Wisconsin-Extension, Cooperative Extension.
The 2 venomous snakes of Wisconsin
Recognize them from a distance; a photo cannot confirm that a snake is safe. Identifying animals safely.
Usually 18 to 30 inches long (New York DEC: 18 to 40 inches, average 27.5). The western massasauga in Arizona reaches only about 22 inches.
How to recognize it
A segmented rattle (a single button in newborns) and vertical, catlike pupils. In Michigan it is the only snake with both.
Nine large plates on top of the head (the timber rattlesnake has many small scales there)
A heat-sensing pit on each side of the head between the eye and nostril; the head is flat and much wider than the neck
Where in Wisconsin
A few isolated populations in the southwestern third of the state, as far east as Walworth County (UW-Extension 2010); DNR 2023: only 9 sites within its historic range University of Wisconsin-Extension, Cooperative Extension
Conservation
federally threatened; state endangered
What the sources say
Venom: “…only two of Wisconsin’s 20-plus snake species are venomous: the timber rattlesnake (Crotalus horridus) and the massasauga, or swamp rattle-snake (Sistrurus catenatus).” (University of Wisconsin-Extension, Cooperative Extension)
Habitat: This rattlesnake is strongly associated with floodplain habitats along medium to large rivers, especially near river confluences, where they primarily occupy open canopy wetlands, such as sedge meadows, fresh wet meadows, shrub-carrs, and adjacent upland prairies, floodplain forests, and old fields. (Wisconsin Department of Natural Resources)
state special concern (fully protected); Protected Wild Animal
What the sources say
Venom: “…only two of Wisconsin’s 20-plus snake species are venomous: the timber rattlesnake (Crotalus horridus) and the massasauga, or swamp rattle-snake (Sistrurus catenatus).” (University of Wisconsin-Extension, Cooperative Extension)
Habitat: Adult males and non-gravid adult females prefer deciduous forests and woodland edges during the summer. Gravid females and juveniles prefer to remain in open-canopy bluff prairies during the summer because of higher preferred body temperatures, but avoid overheating by taking advantage of such things as brush, trees, and rock shelves to provide shade. (Wisconsin Department of Natural Resources)
See all 22 snakes, venomous and not, on Wisconsin snakes. Wisconsin Department of Natural Resources gives Wisconsin 21 snake species in all; that page lists the 22 that official sources name one by one.
Snakes people confuse
Brown Snake (Dekay's Brownsnake) or Red-bellied Snake?
The timber rattlesnake has many small scales on top of the head (the massasauga has nine large plates), a uniformly black tail (the massasauga's tail has 4 to 7 black rings) and narrow, often chevron-shaped back bands rather than bow-tie-shaped blotches. INHS warns that head plates are hard to see on a live snake and says not to get close enough to check, because this is a venomous species.
Massasauga rattlesnake or Fox snake (eastern and western foxsnakes)?
Fox snake (eastern and western foxsnakes) Non-venomous
The fox snake has rounder back blotches (not bow-tie shaped) and only one row of smaller blotches on each side. INHS warns it is not always feasible to rely on the rattle, because segments can break and very young snakes have an inconspicuous rattle.
Massasauga rattlesnake or Common (northern) watersnake?
Ribbon snakes are more slender, have plain (unmarked) lip scales and a longer tail, and their side stripes are on scale rows 3 and 4; on garter snakes the side stripes are on rows 2 and 3.
Photos show typical adults; individuals vary. Never get close to compare.
When venomous snakes are active in Wisconsin
Records of snakes in Wisconsin peak in May to July and September. They show when people record snakes, not how many there are (about month bars).
JanFebMarAprMayJunJulAugSepOctNovDec
590 records of snakes in Wisconsin by month (GBIF, CC0 and CC BY records).
Rattlesnake sightings: call DNR 1-888-WDNRINFO (1-888-936-7463), or use the DNR Rare Species Observation Form / DNR Wildlife Switchboard ([email protected], 608-267-0866). Threatening wolves or wolf depredation: USDA-Wildlife Services (north 1-800-228-1368, south 1-800-433-0663). Spongy moth outside quarantined counties: 1-800-642-MOTH (6684). Other invasive species: DNR Report invasive species page. Feral pig sightings: DNR reporting form or USDA Wildlife Services (north 800-228-1368, south 800-433-0663). Wisconsin Department of Natural Resources, Bureau of Endangered Resources
Venom labels come only from official sources and link to them; "Not stated" never means harmless. How labels work.
Sources
Wisconsin Animals. Wisconsin Department of Natural Resources. undated page; checked November 3, 2026.
Herps of Wisconsin. Wisconsin Department of Natural Resources. undated page; checked November 3, 2026.Quoted passages (2)
“Of Wisconsin's 21 species, 14 are considered "rare" and listed as endangered, threatened or of particular concern.”
“There are two species of rattlesnakes in Wisconsin (timber rattlesnake and eastern massasauga), although both species, especially the eastern massasauga, are rare.”
“…only two of Wisconsin’s 20-plus snake species are venomous: the timber rattlesnake (Crotalus horridus) and the massasauga, or swamp rattle-snake (Sistrurus catenatus).”
“Several nonvenomous Wisconsin snakes, including the bull snake, fox snake, rat snake, and racer, can reach lengths of 4 feet or more.”
“The massasauga has lethal venom and humans have died from its bite, though no record of such deaths exists in Wisconsin.”
“The larger and more dangerous of Wisconsin’s two venomous snakes is the timber rattlesnake, which sometimes reaches lengths of 4½ feet or more.”
“Many non-venomous snakes are often misidentified as rattlesnakes, especially species such as the foxsnake (pine snake) and gophersnake (bullsnake)”
“Seek immediate medical attention”
“Handle, harass, or try to kill the snake; this is how most bites occur”
“The timber rattlesnake is a protected wild animal, meaning it is illegal to collect, possess or kill this species except in cases where there is an immediate threat to humans or domestic animals.”
“Timber rattlesnakes emerge from hibernation as early as April 1 but may continue to emerge well into June. They remain active until as late as mid-October, females that gave birth that year remain active longer than other individuals.”
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