They found that female rats who were kept in solitude were three times more likely than others to go on to develop the cancer.
he tumours the rodents developed were also larger and more deadly.
More than 45,000 women develop the devastating disease every year in Britain.
Reducing feelings of loneliness could potentially cut the numbers affected, the team behind the latest study believes.
Their findings show that levels of a stress hormone called corticosterone increased the rats who were kept in isolation.
The researchers believe that this hormone affects how cells grow, creating tumours.
The study also found that levels of the chemical stayed higher for longer in isolated rats exposed to stressful situations, such as smelling the odour of fox, than in rodents who lived together, the researchers found.
“This study offers insight into how the social world gets under the skin,” said Gretchen Hermes, from Yale University, who led the study.
Martha McClintock, from the University of Chicago and one of the co-authors of the study, added: “We need to use these findings to identify potential targets for intervention to reduce cancer and its psychological and social risk factors.
“In order to do that, we need to look at the problem from a variety of perspectives, including examining the sources of stress in neighbourhoods as well as the biological aspects of cancer development.”
However, cancer charities said that previous studies in humans did not suggest that there was a link between breast cancer and stress.
Ed Yong, from Cancer Research UK, said: “Overall, research in humans does not suggest there is a direct link between stress and breast cancer.
“But it’s possible that stressful situations could indirectly affect the risk of cancer by making people more likely to take up unhealthy behaviours that increase their risk, such as overeating, heavy drinking, or smoking.”
Meg Macarthur, from Breakthrough Breast Cancer, said: “This extremely early stage research is based on a sample of 40 rats – 20 of which were isolated to identify the impact of stress on developing breast cancer.
“This very small study suggests that stress due to isolation might increase the number of breast cancer tumours; however, these findings cannot be directly translated to humans.”
The findings were published in the journal Proceedings of the National Academy of Sciences.