Platycarcinus H. Milne-Edwards, 1834 Cancer is a genus of marine crabs in the family Cancridae.

[2] The genus Cancer, as currently circumscribed, contains eight extant species:[1][3] Three fossil species are also included:[2] As their generic delimitation was based on characters of the dorsal carapace, Schweitzer and Feldmann (2000) were unable to confirm the placement of Cancer tomowoi in the genus, since it is known only from parts of the sternum and the legs.

[2] Other species until recently included in the genus Cancer have since been transferred to other genera, such as Glebocarcinus, Metacarcinus and Romaleon.

[3][4] Linnaeus' cumbersome genus was soon divided into more meaningful units, and Cancer had been restricted to one group of true crabs by the time of Pierre André Latreille's 1802 work Histoire naturelle, générale et particulière des Crustacés et des Insectes ("Natural history in general, and specifically that of crustaceans and insects").

[3] The earliest fossils that can be confidently ascribed to the genus Cancer are those of C. fujinaensis from the Japanese Miocene.