A canine transmissible venereal tumor (CTVT), also known as a transmissible venereal tumor (TVT), canine transmissible venereal sarcoma (CTVS), sticker tumor and infectious sarcoma, is a histiocytic tumor of the external genitalia of the dog and other canines, and is transmitted from animal to animal during mating. It is one of only three known transmissible cancers in mammals; the others are devil facial tumor disease, a cancer which occurs in Tasmanian devils, and contagious reticulum cell sarcoma of the Syrian hamster.

The tumor cells are themselves the infectious agents, and the tumors that form are not genetically related to the host dog. Although the genome of a CTVT is derived from an individual canid (specifically from a population of Native American dogs with coyote contribution), it is now essentially living as a unicellular, asexually reproducing (but sexually transmitted) pathogen. Sequence analysis of the genome suggests it diverged from canids over 6,000 years ago; possibly much earlier. Estimates from 2015 date its time of origin to about 11,000 years ago. However, the most recent common ancestor of extant tumors is more recent: it probably originated 200 to 2,500 years ago.

Canine TVTs were initially described by Russian veterinarian M.A. Novinsky (1841–1914) in 1876, when he demonstrated that the tumor could be transplanted from one dog to another by infecting them with tumor cells.