Like black widows, the female S. grossa is 6-10.5 mm in length, and dark colored, with a round, bulbous abdomen. Typical coloration ranges from purplish-brown to black, with light-colored markings. Unlike black widows, redbacks, and other Latrodectus species, S. grossa does not have a bright red hourglass pattern or any other bright markings. Like many spiders, the male is sometimes smaller; But can many times be nearly as long as the females.
The bite of S. grossa is known to be medically significant in humans, but minor, without any long-lasting effects. Symptoms of bites include blistering at the site of the bite, and/or a general malaise lasting for several days.