A tumor marker is a substance found in the blood, urine, or body tissues that can be elevated in cancer, among other tissue types.

There are many different tumor markers, each indicative of a particular disease process, and they are used in oncology to help detect the presence of cancer. An elevated level of a tumor marker can indicate cancer; however, there can also be other causes of the elevation.

Tumor markers can be produced directly by the tumor or by non-tumor cells as a response to the presence of a tumor.