Scientists at Yale University said even women who had begun exercising for the first time after their diagnosis would experience an improved survival rate.
Dr Melinda Irwin, of the Yale School of Medicine, said: "Anything is better than nothing. We actually observed benefits with just doing a little bit of exercise."
Previous studies have shown that exercise can cut the risk of breast cancer by up to 40%. However, recent research indicates that exercise has an equal or greater impact on survival rates among women with breast cancer.
Dr Irwin and her team looked at 933 women who had been diagnosed with breast cancer between 1995 and 1998.
Researchers found that women who exercised the equivalent of two to three hours of brisk walking per week in the year prior to their breast cancer diagnosis were 31% less likely to die of the disease than those who were sedentary before their diagnosis.
Two years after diagnosis, those who engaged in any amount of exercise had a 64% lower risk of dying than women who were inactive at that time. Women who got at least two to three hours of brisk walking each week reduced their risk of death by 67%, the researchers found.