Hemigrapsus oregonensis is a small shore crab of the family Varunidae; it was formerly classified under the family Grapsidae. It is known under several common names, including yellow shore crab, hairy shore crab, green shore crab, mud-flat crab, and Oregon shore crab.
Despite its name, the body color of this crab can vary. Often, it has a light grey, green or yellow carapace with small blue/black spots and lighter colored legs with similar spots, but it may be red/brown with green spots. The legs with claws (chelipeds) are usually cream/white with no spots.
This species typically lives in mud flats and can be found in large numbers in the San Francisco Bay, and coastal areas of Oregon and Washington states in the United States. Its diet primarily consists of diatoms and green algae, but it will occasionally eat meat.
Although closely related, the adult H. oregonensis is smaller (3.0–3.5 cm or 1.2–1.4 in) than the purple shore crab, H. nudus. It can be distinguished from H. nudus by observing the legs and claws – the legs of H. oregonensis, but not H. nudus, are covered in many hairs called setae, while the claws of H. nudus, but not H. oregonensis, are covered in purple or red spots.