A 53-year-old woman presented to the dermatology clinic with pain, swelling, and discoloration of her right fifth finger approximately 36 hours after receiving a spider bite. She had brought the spider with her to the clinic, and it was identified as a yellow sac spider. On physical examination of the finger, a demarcated, brightly erythematous plaque with a dusky center was observed (Panel A). The patient was treated with acetaminophen and topical mometasone ointment. On reexamination the next day, she had progressive swelling of the digit with an exquisitely tender, purpuric plaque (Panel B). Six days after the bite, the swelling had abated, but there remained an area of dusky purpura with porcelain-white ischemic skin (Panel C). Spiders of the genus cheiracanthium, also known as yellow sac spiders, are common in North America. They are identified by their pale yellow color and dark brown fangs and leg tips. The spiders typically bite persons who are sleeping or dressing, with the bite causing local pain, redness, and swelling, usually without progression to necrosis, as occurred in this case. Two weeks after the bite, the patient’s finger had healed completely.