The report, published in the Journal of Alzheimer's Disease, suggests that the link may be to do with certain biological effects of breastfeeding and that longer periods of breastfeeding lowered the overall risk.
Previous studies have established that breastfeeding can reduce a mother's risk of other diseases but until now little has been done to examine the impact of breastfeeding duration on Alzheimer's risk.
Dr Molly Fox, from the department of biological anthropology at the University of Cambridge, led the study.
She said: "Alzheimer's is the world's most common cognitive disorder and it already affects 35.6 million people. In the future, we expect it to spread most in low and middle-income countries.
"So it is vital that we develop low-cost, large-scale strategies to protect people against this devastating disease."
Although researchers used data gathered from a very small group of 81 British women, they say they observed a highly significant and consistent correlation between breastfeeding and Alzheimer's , although the connection was much less pronounced in women who already had a history of dementia in their family.
The findings may point towards new directions for fighting the global Alzheimer's epidemic - especially in developing countries where cheap, preventative measures are desperately needed.
More broadly, the study opens up new lines of inquiry in understanding what makes someone susceptible to Alzheimer's in the first place.
It may also act as an incentive for women to breastfeed, rather than bottle-feed - which can have wider health benefits for both mother and child.
The study argues that there may be a number of biological reasons for the connection between Alzheimer's and breastfeeding.
One theory is that breastfeeding deprives the body of the hormone progesterone, compensating for high levels of progesterone which are produced during pregnancy.
Progesterone is known to desensitise the brain's oestrogen receptors, and oestrogen may play a role in protecting the brain against Alzheimer's.
Another possibility is that breastfeeding increases a woman's glucose tolerance by restoring her insulin sensitivity after pregnancy.
Pregnancy induces a natural state of insulin resistance and Alzheimer's is characterised by a resistance to insulin in the brain.
"Women who spent more time pregnant without a compensatory phase of breastfeeding therefore may have more impaired glucose tolerance, which is consistent with our observation that those women have an increased risk of Alzheimer's disease," Dr Fox said.
Dr Simon Ridley, head of research at Alzheimer's Research UK, said: "This small study has reported an intriguing association between breastfeeding and Alzheimer's risk but the underlying cause of this association is not clear.
"There could be a range of factors that explain why women who breastfeed also happen to have a lower risk of Alzheimer's. Further research will be needed to investigate whether there is a biological explanation for these findings or to pinpoint other factors that could be involved.
"Preliminary studies like this one are important for highlighting associations worthy of further study, but we shouldn't be quick to jump to conclusions quite yet.
"This study paves the way for further investigations into this association, but research into dementia remains underfunded compared to other conditions. We must continue to invest in research if we are to make progress for the hundreds of thousands of people across the UK living with dementia."