Singletons are around 40 per cent more likely to develop dementia than married people, researchers found.
People who are single into old age or who are widowed are most likely to develop the disease.
Experts suggested this could be because married couples generally tend to be more socially active, have more frequent conversations, eat more healthily and take more physical exercise than people who are single or widowed.
All of these activities have been shown to increase cognitive reserve – which improves brain health and helps protects against dementia.
People who are single into old age or who are widowed are most likely to develop dementia
Lead researcher Dr Andrew Sommerlad, of University College London, said: 'It's not the process of getting married or having a ring on your finger that protects you against the development of dementia.
'But I think what this study tells us is about the lifestyle factors that might come along with marriage which might affect someone's risk of developing dementia.'
How was the research carried out?
Researchers from University College London analysed 14 studies which contained data on more than 800,000 people aged over 65.
They found single people were at a 42 per cent higher risk of developing dementia compared to married people, while widows or widowers were at a 20 per cent higher risk.
But, surprisingly, there was no increased risk for people who were divorced.
The scientists suggested the link was most likely down to lifestyle factors, as they had already taken age and gender into account.
Poor physical health
Single people were statistically more likely to have poor physical health, which is linked to developing dementia.
And widowed people were more likely to have lower levels of education, which is also linked to a higher risk of the disease.
But even once health and education levels were factored in, single people and widows were still 23 and 12 per cent more at risk of dementia than married people.
The pace of modern life means that marriage may no longer be a defence against ill-health.
Research published last week showed the once-wide gap in the wellbeing of those who marry and those who do not has vanished.
One theory is couples now spend less time together and are