Steatoda nobilis is a spider in the genus Steatoda, known in the United Kingdom as the noble false widow, as it superficially resembles and is frequently mistaken for the dangerous black widow and other spiders in the genus Latrodectus. It is often referred to as the false widow, although "false widow" is a more general term applied to a wider group of species with this resemblance. It is a medically significant spider, where in England it has a reputation as one of the few local spider species that is capable of inflicting a painful bite to humans, with most bites resulting in symptoms similar to a bee or wasp sting, and some causing more significant harm, partly due to pathogenic bacteria from the spiders. S. nobilis is spotted all year round, both indoors and outdoors in a variety of habitats including cacti, roadside cuttings, and demolished buildings. These spiders prey on both invertebrates and small vertebrates using an "attack wrap" strategy where silk is wrapped around the victim. Steatoda nobilis is native to Madeira and the Canary Islands from where it is thought to have spread to Europe, and continued to spread to other parts of the world including the United States, Chile, Colombia. They are one of the world's most invasive species of spider.

Steatoda nobilis has a brown bulbous abdomen with cream coloured markings that are often likened to the shape of a skull. Their legs are reddish-orange. Both female and male S. nobilis can be distinguished from other spiders of the same genus by their large size and typical colouration. Females range in size from about 9.5 to 14 mm in size, while males are 7 to 11 mm. The largest females are 13.7 mm in size, while the largest males can be 11.66mm in size. The males can be distinguished by their conformation of the palp and by their ventral abdominal markings. The females can be distinguished by their epigyne. The variation in size, shape and markings that have been observed is not thought to be due to location. Spiders found centimeters apart and siblings born from the same egg sac can look very different. Male and female juvenile spiders are indistinguishable from each other.

The spider is an introduced species across Europe, plus parts of North Africa, and likely spreading. It was found for the first time in 2011 in Cologne, Germany. It is originally from the Canary Islands and Madeira. In England it has been reported mostly in southern counties, but its range was reported to be expanding northwards in the 21st century. In 2011, the spider was reported as an established invasive species in the USA, in Ventura County, California. In January 2016, it was reported that Steatoda nobilis had been found in Chile, the first time that the species had been recorded in the southern hemisphere. Research published in December 2018 showed that it was also established in Colombia and Ecuador. It is regarded to be one of the world's most invasive species of spiders. Seaside cities and villages with a temperate climate are especially favorable habitats for this spider. The first recorded observation of S. Nobilis in England dates back to 1879. The first recorded observation of S. Nobilis in Ecuador was in 2014 at several locations as high as 2800 meters. Between 1985-2010 there was not high reportings of S. Nobilis anywhere, and scientists consider this to be a typical lag phase that is a phenomenon observed in many invasive species. Afterwards there was a large accelerated spread observed all over the globe. They are able to establish in urban environments and build large populations in a short time.

The distribution of Steatoda nobilis is expected to increase northwards in the UK, due to, at least partly, mild winters in recent years. This prediction was reported by Stuart Hine of the Natural History Museum, and is substantiated by the National Recording Scheme. The spider is reported to be an established species in Ireland. The first recorded sighting of the species occurred in the east of Ireland in Bray, Co. Wicklow in 1997.

Steatoda nobilis uses an effective "attack wrap" strategy to immobilize would-be prey or predators, meaning that they are in close contact to their prey/predator. Prey is captured in typical theridiid fashion, where silk is wrapped around the victim using the spider’s fourth legs, allowing the spider to bite the victim. Their venom allows them to immobilize their prey by inducing paralysis. S. nobilis have been observed biting insects and spiders which causes a rapid reduction in motor function, most likely due to the release of venom. Their potent venom allows them to capture vertebrates much larger than themselves; the spider has been observed feeding on pipistrelle bats.

S. nobilis can eat both vertebrates and invertebrates. In Ireland, they were observed to eat woodlice. All of S. nobilis’s liquid requirements are observed to come from its prey. In laboratories they seem to thrive without water and in extremely dry conditions. In Ireland they have been observed to prey on protected reptile species.

As with other members of the family Theridiidae, Steatoda nobilis constructs a cobweb which is an irregular tangle of sticky silken fibres. Its 'scaffold web' differs from others of the genus in the exceptional strength of the silk, and in the tubular retreat that is at least partly concealed in a deep crack or hole. They produce three dimensional cob webs. The web is developed slowly, starting as a typical theridiid tangled web and gradually takes on the characteristic form over several days. They have poor eyesight and depend mostly on vibrations reaching them through their webs to orient themselves to prey or warn them of larger animals that could injure or kill them.

Before oviposition, the female produces an irregular, silky brooding chamber that does not have an entrance. They spend about two to ten days in the chamber producing the egg sacs. The egg sacs are spherical or pear-shaped. When the egg sacs are produced a first layer of loose silk is wrapped around the eggs and then a second layer of denser silk is added. The egg sacs are about 5-12mm in diameter and are suspended by threads inside the brooding chamber. Each egg sac contains 94 eggs on an average but the numbers can range from 34 to 208 eggs. The spiderlings will emerge from their chorion 18 days after egg production, and remain within the egg sac. After the first molt, the spiderlings will emerge from the egg sac. At this point they are able to capture and consume small live prey. The period of time between mating, egg-laying and emergence of spiderlings can vary, and is temperature dependent. They are able to build several egg sacs within one season, and one study found that one female spider produced four egg sacs within four months. S. nobilis can produce large amounts of offspring for a long period after mating.

The S. nobilis spiders have a high longevity, with a lifespan of up to five years. They have a fast reproductive rate and are cold tolerant with year-round activity. Adult females are observed to be extraordinarily long-lived and persistent. One adult S. nobilis female was reported to live almost five and a half years in captivity.

After locating a female’s web, Steatoda nobilis males pluck or tap the web in a rhythmic way with the second pair of legs. This is accompanied by brief bursts of abdominal vibrations, during which the male uses his first pair of legs to search for the female. If the legs make contact with the female, the male will repeatedly jab his palps into the epigyne until the palps are inserted. Males are able to produce stridulation sounds during courtship, by scraping 10-12 teeth on the abdomen against a file on the rear of the carapace.

S. nobilis presents a unique risk because of their synanthropic habits, especially in temperate regions, that bring them in close contact to humans. They are considered one of the most dangerous spiders in Western Europe. The mechanical bite from an adult S. nobilis is usually painless. It is the release of venom that causes the intense pain commonly reported. Male bites are less severe than those of females.

Two thirds of the venom is composed of latrodectus-like toxins. Their venom is mainly composed of peptidase, serine protease, alpha latrotoxin and delta lactroinsectotoxin. Peptidase and serine protease are both pancreatic lipases and chitinases (enzymes) that help with digestion. Alpha-latrotoxin and delta latroinsectotoxin are toxins. About 49% of the venom is toxins, 15% are enzymes, 18% are proteins with other functions and 18% are proteins with unknown functions.

Alpha-latrotoxin works by creating a pore in the neurons and allowing an influx of Ca2+. This triggers an efflux of neurotransmitters, and once all the neurotransmitters leave, the signals between nerve and muscles are blocked. This leads to neuromuscular paralysis.

The venom of S. nobilis serves two functions: immobilization of prey or predators and the predigestion of prey. This spider is able to efficiently and safely capture prey that are often times much larger or stronger than them via the rapid paralysis/ immobilization tactic. Their venom is fast acting and can subdue both invertebrate and vertebrate prey. The range of enzymes in the venom suggest that the venom also functions to pre-digest the prey. One example is the enzyme Chitinase which can breakdown the exoskeleton (shell) of arthropods.

In December 2020, a study by scientists at NUI Galway determined that the noble false widow vectors pathogenic bacteria, and that some are antibiotic resistant. It carries bacteria on its chelicerae and the surface of its body. Twelve pathogenic bacterial species were isolated, including K. intermedia and P. putida. This showed that infections may be due to the spider bite directly, rather than opportunistic bacteria present on the skin.

The symptoms of a bite are typically similar to a bee or wasp sting. The bite of this spider, along with others in the genus Steatoda, can produce a set of symptoms known as steatodism. Symptoms of bites include intense pain radiating from the bite site, along with feverishness or general malaise. Other symptoms observed in humans after envenomation include prolonged, moderate to intense pain, swelling, erythema, piloerection, diaphoresis, facial flushing, feverishness, vasodilation of capillaries localized near the site of the bite.

Sensationalised stories about the bite of Steatoda nobilis have featured in UK newspaper articles. Stuart Hine from the Natural History Museum, London responded on the naturenet blog, stating, "Of course I also explain the great value of spiders and how rare the event of spider bite in the UK actually is. I also always explain that up to 12 people die from wasp/bee stings in the UK each year and we do not panic so much about wasps and bees – but this never makes it past editing." Steven Falk, an entomologist, cautioned that without "hard evidence", it is difficult to know how many of the bites reported in the media have been caused by false widow spiders. The arrival of Steatoda nobilis in Chile in 2016 led to a similar media reaction.

- In 2006 a Dorchester man spent three days in Dorset County Hospital with symptoms of heart seizure, after suffering a spider bite believed to be caused by Steatoda nobilis. - In 2013 a man in Sidcup, Kent was allegedly bitten in his sleep, reporting that his hand had turned black and yellow. His hand remained swollen for five weeks until doctors gave him a course of antibiotics. - In October 2013, it was reported that a man from Romford in London had been allegedly bitten by a false widow. He was treated for bacterial infection with antibiotics and needed to have his leg drained of pus. - In October 2013, a British school in the Forest of Dean was closed for a day for fumigation as a result of a dense population of Steatoda nobilis on the site. - In 2014, a woman from County Durham had her left index finger amputated after contracting the flesh-eating bug necrotising fasciitis following a claimed bite from a false widow spider. - In October 2014, an Irish man went into cardiac arrest and spent a day in intensive care after being bitten three times by what was claimed as a false widow, on the hip, side and shoulder. - In October 2018 four east London schools were closed due to false widow spider infestations. - In September 2019, it was reported that a man in Southampton was bitten while he slept, and left in pain, unable to work, and "barely able to walk" due to an allergic reaction to the venom.

a. The full English name for Steatoda nobilis is "noble false widow". Media coverage usually abbreviates this to "false widow", although Steatoda nobilis is one of several false widows; Steatoda grossa and Steatoda paykulliana are other examples of false widow spiders.