Experts said “modest” weight loss significantly cuts the chances of developing the disease.
Six in ten British women are overweight or obese, with the highest rates of obesity among those in middle age.
Excess weight raises the risk of breast cancer as fat cells produce hormones that help drive tumours.
But until now, it was not clear whether the risk could be reduced, by a succesful diet. Around 50,000 women are diagnosed with breast cancer each year – with 12,000 dying.
And four in five cases occur in women after they have gone through the menopause.
The US research tracked more than 61,000 women aged between 50 and 79 for 11 years.
It found those who lost five per cent of their weight saw their breast cancer risk fall by 12 per cent.
And the risk fell by 37 per cent for postmenopausal women who slimmed down by 15 per cent.
For a 12 stone woman who is five foot five inches tall, this would mean losing almost two stone.
Lead researcher Dr Rowan Chlebowski, from the Department of Medical Oncology and Therapeutics Research at City of Hope in Duarte, California, said: “Relatively modest weight loss was associated with significant lowering of breast cancer incidence.
“From this study, we have evidence that a weight loss strategy can be effective in lowering breast cancer risk in postmenopausal women.”
The findings were presented at the San Antonio Breast Cancer Symposium.
Baroness Delyth Morgan, Chief Executive at Breast Cancer Now, said action on the findings could save lives.
She said: “This important study provides further, clear evidence that postmenopausal women can significantly reduce their risk of breast cancer by taking steps to lose weight.
“Breast cancer risk increases with age. Being overweight after the menopause does increase your risk of the disease, likely because fat tissue becomes a women’s main source of oestrogen after the menopause.
“The less body fat you have, the lower your oestrogen levels, which can decrease your breast cancer risk.” Improved fitness and cutting down alcohol could also reduce the risk of breast cancer, she added.