The average length of a Spotted Owl is 45 cm, with a wingspan of 114.3 cm. The average length of the left and right ear openings are 17.5 mm and 22 mm, respectively. Weight ranges from 518 to 760 grams.
Females are generally larger than males, and have a higher pitched call.
These owls are medium-sized with dark brown plumage, a round head, and large dark eyes. The head and hind neck have white spots, along with white mottling on the breast and abdomen. Depending on how thick the plumage is, the amount of coloration and white spotting varies. Birds from the humid climate of the coastal range are darkest, while those from the mountain ranges in Arizona and Mexico are the lightest with the most white spots. This difference in appearance is assumed to be an evolutionary adaptation to climate variation.