Ovarian cancer is a cancerous growth arising from an ovary. Although ovarian cancer is known to occur in many species, the majority of the medical literature and the focus of this article is on ovarian cancer in humans.

Ovarian cancer is thought to most commonly form in the outer lining of the ovary (resulting in epithelial ovarian cancer) and, less commonly, in the egg cells (resulting in a germ cell tumor). However, recent evidence points towards the Fallopian tube as precursor for ovarian cancers

Ovarian cancer
is the fifth leading cause of death from cancer in women and the leading cause of death from gynecological cancer. A woman has a lifetime risk of ovarian cancer of around 1.5%, which makes it the second most common gynecologic malignancy (the first being breast cancer).

Ovarian cancer has been called "the silent killer" because it frequently causes non-specific symptoms, which contribute to diagnostic delay, resulting in a late stage and a poor prognosis. Most women with ovarian cancer report one or more symptoms such as abdominal pain or discomfort, an abdominal mass, bloating, back pain, urinary urgency, constipation, tiredness and a range of other non-specific symptoms, as well as more specific symptoms such as pelvic pain, abnormal vaginal bleeding or involuntary weight loss. There can be a build-up of fluid in the abdominal cavity (this is called ascites).

An abnormal physical examination (including a pelvic examination), a blood test (for CA-125, more specifically) or medical imaging studies can provide evidence leading to an ovarian cancer diagnosis. The diagnosis can be confirmed with a surgical procedure (open or keyhole surgery) to inspect the abdominal cavity, take biopsies (tissue samples for microscopic analysis) and look for cancer cells in the abdominal fluid. Treatment usually involves chemotherapy and surgery, and sometimes radiotherapy.