Achaeus japonicus, sometimes known as the orang-utan crab, is a crab of the family Inachidae (spider crabs or decorator crabs) which can be observed in tropical waters of the central Indo-Pacific. With a carapace of only about 2 centimetres or 0.8 inches in diameter, it has relatively long arms, which are thickly covered with fine hairs, red or reddish brown in colour, and often laden with small bits of debris for further camouflage. It is frequently, but not always, found in association with the bubble coral Plerogyra sinuosa.