Seattle is home to hundreds of spider species, but the ones you’re most likely to notice indoors are a small handful of large, fast, and mostly harmless house spiders. Washington state hosts at least 970 spider species overall, yet only two are considered medically significant, and neither is common in the Seattle area. If you’ve spotted a big brown spider sprinting across your basement floor, here’s what you’re dealing with.
The Giant House Spider
The spider most Seattle residents encounter, and the one that tends to alarm people, is the giant house spider. These brown to grayish-brown spiders have a body length of roughly half an inch to one inch, but their leg span can reach up to four inches in large adults. They’re hairy, not shiny, and their coloring varies widely from light to dark with markings that range from barely visible to very distinct.
Giant house spiders are found exclusively west of the Cascade Mountains in Washington, which puts Seattle squarely in their territory. They prefer dark spaces and are well adapted to indoor life, turning up most often in garages, basements, and crawl spaces. They’re not great climbers, so you’ll usually find them on the floor or trapped in sinks and bathtubs where they wandered looking for water. When startled, they run away remarkably fast, which is the main reason people find them unsettling. They are not aggressive and pose no medical risk.
The Hobo Spider Myth
For years, the hobo spider had a reputation as one of the Pacific Northwest’s dangerous spiders. It was even nicknamed the “aggressive house spider.” Both labels are wrong. The Washington State Department of Health states plainly: hobo spiders are not considered dangerous. They are shy, reluctant to bite, and try to escape when disturbed. There is no evidence that hobo spider venom causes skin necrosis or necrotic wounds in people.
Hobo spiders are found throughout Washington and look similar to giant house spiders, which causes confusion. The hobo spider is noticeably smaller, with females measuring roughly 3/8 to 5/8 of an inch in body length. Its abdomen has a pattern of chevron-shaped yellowish markings on a gray background, though these can be hard to see without magnification. One useful distinction: hobo spiders lack the dark bands on their legs that many similar species have. But color alone isn’t reliable for identification because both species vary so much in appearance.
Telling the two apart with certainty requires a microscope. Arachnologists look at features like the number of prongs on the male’s reproductive appendages (two for hobo spiders, one for giant house spiders) or the feather-like hairs shared by all funnel-web spiders. For practical purposes, the distinction doesn’t matter much, since neither species is dangerous.
Spiders That Actually Matter Medically
Washington has two spiders of medical significance: the black widow and the yellow sac spider. Neither is a major concern for most Seattle residents, but both deserve a mention.
Black widows are common in eastern Washington. Only a few small populations exist on the western side of the state, making encounters in Seattle rare. The yellow sac spider, however, is found in the Seattle area. It’s small, with a body only about a quarter to a half inch long, and pale yellowish-green. Yellow sac spiders can deliver a painful bite that may cause localized redness and swelling, but serious reactions are uncommon.
Why Spiders Invade in Late Summer
If it seems like spiders take over your house every August, you’re not imagining it. Male giant house spiders and hobo spiders reach maturity in August and September, and once mature, they abandon their webs and start roaming in search of females. This is when they end up crossing your living room floor or appearing in the bathtub. Mating season typically wraps up by October, and sightings drop off sharply after that.
The spiders were likely already living in your home year-round, tucked away in undisturbed corners of the garage or basement. You’re simply seeing the males because they’ve started moving. They’re not migrating in from outside in most cases.
Other Spiders You Might See
Beyond the house spiders, Seattle yards and gardens support a wide variety of species. Wolf spiders are common ground hunters that don’t build webs, instead chasing prey across open ground. You might spot the banded garden spider, a striking orb weaver, in late summer garden beds. Crab spiders like the flower crab spider sit motionless on flowers waiting to ambush pollinators, and their ability to shift color between white and yellow makes them easy to overlook.
Indoors, you may also encounter cobweb spiders from the family that includes the common house spider (not to be confused with the giant house spider). These are small, build messy webs in corners and window frames, and are completely harmless. The vast majority of the 970-plus species in Washington live in leaf litter, mosses, tree canopies, and other outdoor habitats where most people never notice them.
Keeping Spiders Out of Your Home
The most effective spider control is also the simplest: regular cleaning. A vacuum cleaner or broom removes spiders, webs, and egg sacs all at once. Reducing clutter in basements, garages, and storage areas makes those spaces less appealing, since spiders prefer undisturbed hiding spots.
Physical exclusion is worth the effort if you’d rather not share your space with them. Install tight-fitting window screens and door sweeps, paying particular attention to door thresholds, garage entrances, crawl space openings, foundation vents, and the bottom edge of siding. Outside, move firewood, stacked items, and debris away from your foundation. Trim shrubs, vines, and tree limbs that touch the house, since these give spiders a direct bridge to the structure.
Pesticides are rarely necessary for spider control in Seattle. The spiders you’re finding are beneficial predators that eat flies, mosquitoes, and other insects. If you can tolerate them in the garage or basement, they’re earning their keep.