Notomithrax ursus, known as the hairy seaweed crab, is a spider crab of the family Majidae, found in south-east Australia and New Zealand. Males grow to a carapace width of 42 mm (1.7 in), and a carapace length of 60 mm (2.4 in), while females reach 36 mm (1.4 in) by 43 mm (1.7 in). They are typically found among seaweed in rockpools, and are usually covered in seaweed.