Swedish research, presented at a US conference, found that marriage or having a partner halved the risk of developing dementia.
Scientists believe social interaction between couples may ward off illness.
The Alzheimer's Research Trust said the results were worrying, given the high divorce rates in the UK.
These findings are particularly worrying for the UK - a society with a high divorce rate, marriage at an all-time low, and ageing population.
The study by the Karolinska Institute suggested that the problem might be even greater for some people.
Divorcees who remained single, they noticed, had a trebled risk of dementia, while those widowed at a young age who stayed single faced a six times greater chance.
The research looked at 1,449 people from a Finnish database, who were asked about their relationship status in mid-life, then revisited 21 years later to see if they had developed dementia.
In total, 139 of them had some sort of cognitive impairment, and Alzheimer's had been diagnosed in 48 of these.
Even after other factors which might have an impact on dementia were adjusted for, the study consistently showed people with partners as less prone to the illness.
"Living in a couple relationship is normally one of the most intense forms of social and intellectual stimulation. If social and cognitive challenges can protect against dementia, so should living as a couple," BBC quoted Dr Krister Hakansson, the study's lead researcher, as saying.