The body of the female, consisting of two regions (cephalothorax and abdomen), is typically shiny black with red markings. The red is usually an hourglass-shaped mark on the underside (venter) of the abdomen, but this may be reduced to remnants. Usually there is a single red spot just behind the spinnerets, sometimes a row along the back. There is much variation in body color with southern and western specimens being more strikingly marked than northern and eastern ones. The male has the abdomen narrower, with white lines along the sides which is shown by immatures as well.

Young spiderlings are orange and white, and acquire more black in later developmental stages (instars) until some have little or no red except the hourglass markings. Immatures have the abdomen gray with white curved stripes. The body length of the female is about 3/8 inch and that of the male is about 3/16 inch, but sizes within different geographical populations can vary widely. There are more than 25 Latrodectus species world wide. Contrary to common belief, the female does not consume the male in most situations, except when held together in cages from which the male cannot escape. Examination of the genitalia is the proper way to identify species in this group.