Amarinus lacustris is a species of freshwater crab from Australia, New Zealand and nearby islands, where it lives in water of various salinities.

[4] In New Zealand, A. lacustris is the only freshwater crab, and is found in a variety of habitats, including lakes, slow-flowing streams and larger rivers.

It is found from the lower reaches of the Waikato River to Northland, with a notable population in the Oratia Stream near Auckland.

[5][6] The carapace is grey-brown and almost circular, marked with a set of grooves forming an H-shape with the two uprights bending outwards, and the whole animal is covered in setae.

[8] Amarinus lacustris lives in estuarine, freshwater and shallow subtidal habitats, usually in salinities of 1–6‰, but occasionally to 0.1–9.6‰.