Having both raised the risk 14 times compared with the rate for women without diabetes.
On their own, high oestrogen levels also increased the chances of dementia, but not by as much.
Scientists measured oestrogen in the blood of 5,644 post-menopausal women aged 65 or older who did not have dementia at the time.
Four years later, the women were given a further assessment and 132 were found to have dementia.
Both very low and very high levels of natural oestrogen doubled the chances of dementia compared with normal levels, the researchers said.
But the risk was amplified in 10 women who had both diabetes and high levels of oestrogen.
It was 14 times greater than it was for women with diabetes and normal hormone levels.
The findings, from the French National Institute of Health and Medical Research, appeared in the journal Neurology.
Dr Doug Brown, director of research at the Alzheimer's Society, said: "It is surprising to see that older women with both naturally high oestrogen levels and diabetes were so much more likely to develop dementia in this study.
"However, as this only applied to 10 women, we need to conduct much larger studies before reaching any conclusions.
"Although it's very difficult to do anything about lowering your oestrogen levels, we do know that people can take steps to reduce their chance of developing diabetes.
"Watching what you eat and taking regular exercise might be the key to helping prevent both diabetes and dementia."