Colorectal cancer, also known as bowel cancer, colon cancer, or rectal cancer, is the development of cancer from the colon or rectum (parts of the large intestine). It is the consequence of uncontrolled growth of colon cells that can invade/spread to other parts of the body. Signs and symptoms may include blood in the stool, a change in bowel movements, weight loss, abdominal pain and fatigue. Most colorectal cancers are due to lifestyle factors and genetic disorders. Risk factors include diet, obesity, smoking, and lack of physical activity. Dietary factors that increase the risk include red meat, processed meat, and alcohol. Another risk factor is inflammatory bowel disease, which includes Crohn's disease and ulcerative colitis. Some of the inherited genetic disorders that can cause colorectal cancer include familial adenomatous polyposis and hereditary non-polyposis colon cancer; however, these represent less than 5% of cases. It typically starts as a benign tumor, often in the form of a polyp, which over time becomes cancerous.

Colorectal cancer may be diagnosed by obtaining a sample of the colon during a sigmoidoscopy or colonoscopy. This is then followed by medical imaging to determine whether the cancer has spread beyond the colon or is in situ. Screening is effective for preventing and decreasing deaths from colorectal cancer. Screening, by one of several methods, is recommended starting from ages 45 to 75. It was recommended starting at age 50 but it was changed to 45 due to increasing numbers of colon cancers. During colonoscopy, small polyps may be removed if found. If a large polyp or tumor is found, a biopsy may be performed to check if it is cancerous. Aspirin and other non-steroidal anti-inflammatory drugs decrease the risk of pain during polyp excision. Their general use is not recommended for this purpose, however, due to side effects.

Treatments used for colorectal cancer may include some combination of surgery, radiation therapy, chemotherapy, and targeted therapy. Cancers that are confined within the wall of the colon may be curable with surgery, while cancer that has spread widely is usually not curable, with management being directed towards improving quality of life and symptoms. The five-year survival rate in the United States was around 65% in 2014. The chances of survival depends on how advanced the cancer is, whether all of the cancer can be removed with surgery, and the person's overall health. Globally, colorectal cancer is the third-most common type of cancer, making up about 10% of all cases. In 2018, there were 1.09 million new cases and 551,000 deaths from the disease (Only colon cancer, rectal cancer is not included in this statistic). It is more common in developed countries, where more than 65% of cases are found.

The signs and symptoms of colorectal cancer depend on the location of the tumor in the bowel, and whether it has spread elsewhere in the body (metastasis). The classic warning signs include: worsening constipation, blood in the stool, decrease in stool caliber (thickness), loss of appetite, loss of weight, and nausea or vomiting in someone over 50 years old. Around 50% of people who have colorectal cancer do not report any symptoms.

Rectal bleeding or anemia are high-risk symptoms in people over the age of 50. Weight loss and changes in a person's bowel habit are typically only concerning if they are associated with rectal bleeding.

75–95% of colorectal cancer cases occur in people with little or no genetic risk. Risk factors include older age, male sex, high intake of fat, sugar, alcohol, red meat, processed meats, obesity, smoking, and a lack of physical exercise. The Rectal Cancer Survival Calculator developed by the MD Anderson Cancer Center additionally considers race to be a risk factor; however, there are equity issues concerning whether this might lead to inequity in clinical decision making. Approximately 10% of cases are linked to insufficient activity. The risk from alcohol appears to increase at greater than one drink per day. Drinking five glasses of water a day is linked to a decrease in the risk of colorectal cancer and adenomatous polyps. The consumption of dairy products, such as milk, is protective against colorectal cancer. Streptococcus gallolyticus is associated with colorectal cancer. Some strains of Streptococcus bovis/Streptococcus equinus complex are consumed by millions of people daily and thus may be safe. 25 to 80% of people with Streptococcus bovis/gallolyticus bacteremia have concomitant colorectal tumors. Seroprevalence of Streptococcus bovis/gallolyticus is considered as a candidate practical marker for the early prediction of an underlying bowel lesion at high-risk population. It has been suggested that the presence of antibodies to Streptococcus bovis/gallolyticus antigens or the antigens themselves in the bloodstream may act as markers for the carcinogenesis in the colon.

Pathogenic Escherichia coli may increase the risk of colorectal cancer by producing the genotoxic metabolite, colibactin.

People with inflammatory bowel disease (ulcerative colitis and Crohn's disease) are at increased risk of colon cancer. The risk increases the longer a person has the disease, and the worse the severity of inflammation. In these high risk groups, both prevention with aspirin and regular colonoscopies are recommended. Endoscopic surveillance in this high-risk population may reduce the development of colorectal cancer through early diagnosis and may also reduce the chances of dying from colon cancer. People with inflammatory bowel disease account for less than 2% of colon cancer cases yearly. In those with Crohn's disease (with colonic involvement), 2% get colorectal cancer after 10 years, 8% after 20 years, and 18% after 30 years. In people who have ulcerative colitis, approximately 16% develop either a cancer precursor or cancer of the colon over 30 years.