Bacillus Calmette–Guérin (BCG) vaccine is a vaccine primarily used against tuberculosis (TB). It is named after its inventors Albert Calmette and Camille Guérin. In countries where tuberculosis or leprosy is common, one dose is recommended in healthy babies as soon after birth as possible. In areas where tuberculosis is not common, only children at high risk are typically immunized, while suspected cases of tuberculosis are individually tested for and treated. Adults who do not have tuberculosis and have not been previously immunized, but are frequently exposed, may be immunized, as well. BCG also has some effectiveness against Buruli ulcer infection and other nontuberculous mycobacterial infections. Additionally, it is sometimes used as part of the treatment of bladder cancer.

Rates of protection against tuberculosis infection vary widely and protection lasts up to 20 years. Among children, it prevents about 20% from getting infected and among those who do get infected, it protects half from developing disease. The vaccine is given by injection into the skin. No evidence shows that additional doses are beneficial.

Serious side effects are rare. Often, redness, swelling, and mild pain occur at the site of injection. A small ulcer may also form with some scarring after healing. Side effects are more common and potentially more severe in those with immunosuppression. Although no harmful effects on the fetus have been observed, there is insufficient evidence about the safety of BCG vaccination during pregnancy and therefore, vaccine is not recommended for use during pregnancy. The vaccine was originally developed from Mycobacterium bovis, which is commonly found in cattle. While it has been weakened, it is still live.

The BCG vaccine was first used medically in 1921. It is on the World Health Organization's List of Essential Medicines. As of 2004, the vaccine is given to about 100 million children per year globally.