The Sydney funnelweb spider, Atrax robustus, is an Australian funnelweb spider found primarily around Sydney, Australia. It is a notoriously aggressive species and is among the deadliest spiders in the world.

Sydney funnelwebs are medium-to-large in size, with body lengths ranging from 2 cm to 7 cm (0.9" to 3"). They are glossy and darkly coloured, ranging from blue-black to black to brown or dark plum coloured. The carapace covering the cephalothorax is almost hairless and so appears smooth and glossy.

It is the only species in the genus Atrax, contained by the family Hexathelidae. The common name of the species is shared by others, members of the genus Hadronyche, but it is the only one of those termed as Australian funnel-web spiders known to be fatal.[1] The species is not closely related to the Araneomorphae, and are described within the Mygalomorphae infraorder of Araneae.[2]

The spider is highly aggressive if provoked. The long-lived female funnel-webs spend most of the time in their silk-lined tubular burrow retreats. Males, recognized by the modified terminal segment of the palp, tend to wander during the warmer months of the year looking for receptive females of their kind for mating.[3] They are attracted to water and hence are often found in swimming pools where they have fallen while wandering. The spiders can survive such immersion for up to twenty-four hours and can deliver a bite when removed from the water.[4] They also show up in garages and yards in suburban Sydney.

They are mostly terrestrial spiders, that favour habitat with moist sand and clays. They typically build silk-lined tubular burrow retreats with collapsed "tunnels" or open "funnel" entrances from which irregular trip lines radiate out over the ground. In some exceptions, which lack trip lines but may have trapdoors, the silk entrance tube may be split into two openings, in a Y or T form. The burrow may be in the hollow of a tree trunk or limb, many meters above ground level.[5]