While it is well known that being active cuts risk of the disease, this is the first study to specifically look at the effect of walking.
The American Cancer Society study involved 73,615 postmenopausal women, of whom 4760 were diagnosed with breast cancer during a 17-year follow-up.
Among all women in the group, 47% said walking was their only recreational activity.
Those who walked at least seven hours per week had a 14% lower risk of breast cancer compared to those who walked three or fewer hours per week.
The study also found that women who took part in more vigorous activities for an hour a day had a 25% lower risk of breast cancer than the least active, which echoed findings in other studies. The results were unaffected by factors such as weight or hormone replacement therapy.
Dr Alpa Patel, senior epidemiologist at the American Cancer Society in Atlanta, Georgia, said: "Our results clearly support an association between physical activity and postmenopausal breast cancer, with more vigorous activity having a stronger effect.
"Just walking an average of one hour per day was associated with lower risk of breast cancer."