It is one of many species in the genus Latrodectus, such as the Australian redback (L. hasseltii), and the North American black widow.

In black katipō females found in the upper half of the North Island, this stripe is absent, pale, yellow, or replaced with cream-coloured blotches.

The male is much smaller than the female and quite different in appearance: white with black stripes and red diamond-shaped markings.

Katipō feed mainly on ground dwelling insects, caught in an irregular tangled web spun amongst dune plants or other debris.

After mating in August or September, the female katipō produces five or six egg sacs in November or December.

A katipō bite produces the toxic syndrome latrodectism; symptoms include extreme pain and, potentially, hypertension, seizure, or coma.

The katipō is particularly notable in New Zealand as the nation is almost entirely devoid of dangerous native wildlife; this unique status means the spider is well known, despite being rarely seen.

[5] The katipō is so closely related to the redback that it was at one stage thought to be a subspecies, with the proposed name Latrodectus hasseltii katipo.

The red katipō female, found in the South Island and the lower North Island, has a large black globular abdomen, about the size of a garden pea, with slender legs and a white-bordered orange or red stripe on its back that runs from the uppermost surface of the abdomen back to the spinnerets.

[13] Variations also exist where the abdomen, cephalothorax, or entire body is brown, sometimes with a dull red or yellow stripe, or cream-coloured spots on its upper side.

Juveniles have a brown carapace, with a predominantly white abdomen which has a series of red-orange diamonds running along the dorsal region bordered on either side by irregular black lines.

[7][16][19][20] Due to its much smaller size, Urquhart (1886) believed the male to be a separate species and named it Theridion melanozantha.

They generally reside on the landward side of dunes closest to the coast where they are most sheltered from storms and sand movement.

[8] Webs are typically established in low-growing dune plants and other vegetation such as the native pīngao (Ficinia spiralis) or the introduced marram grass (Ammophila arenaria).

Marram grass has been extensively planted in New Zealand to help stabilise sand dunes and has largely replaced pīngao in many areas.

Because marram grass grows in a very tight formation only leaving small gaps between tuffs, this makes it difficult for the katipō to construct a suitable web for capturing prey.

[15] The web consists of a broad base with many supporting threads above and below, including a number of sticky guy lines anchored to debris in the sand.

Like most theridiids, the tarsi of the hind legs have a row of strong curved bristles which are arranged as a comb.

The katipō uses these to scoop sticky silk from its spinnerets and throws it over the insect with a series of rapid movements.

The courtship process consists of the male bobbing, plucking and tweaking the web along with periods of cautious approach and being chased by the female.

[14] After mating, the male retreats to groom, which is performed by running his palps and legs through his fangs and wiping them over his body.

In one study, observing spiders over 24 hours, 28% used a ballooning method, which is where the young spiders use air currents to carry themselves away from the nest suspended by a single web strand, while the majority, 61%, used a bridging method where the spiderling uses its silk to move to nearby plants, and 11% still remained in the nest.

It was first reported in the 1990s and may have displaced the katipō along the west coast of the North Island from Wellington to Whanganui,[11][24] although both the katipō and S. capensis have been found sharing the same dune systems or even co-existing under the same piece of driftwood, suggesting that the two species can co-exist in similar habitats.

[13] In 2010 the katipō was one of a dozen species of previously unprotected invertebrate given full protection under the 1953 Wildlife Act, noted as "iconic, vulnerable to harm, and in serious decline".

[18] The katipō will only bite as a last resort; if molested, the spider will usually fold up into a ball and drop to the ground or retreat to the nearest cover.

[18] The venoms of all Latrodectus spiders are thought to contain similar components with the neurotoxin α-latrotoxin being the main agent responsible.

Less commonly, systemic envenoming is heralded by swollen or tender regional lymph nodes; associated features include malaise, nausea, vomiting, abdominal or chest pain, generalised sweating, headache, fever, hypertension and tremor.

[30][36] Rare complications include seizure, coma, pulmonary edema, respiratory failure or localised skin infection.

Hospital assessment is recommended if simple analgesia does not resolve local pain or clinical features of systemic envenoming occur.

[18] Redback antivenom can also cross-neutralise katipō venom,[42] and it is used to treat envenoming from Latrodectus katipo in New Zealand.