US researchers led by Dr Maria Norton from the University of Utah studied 1,221 married couples (2,442 individuals) aged 65 or older.
During 12 years of study, 255 of the participants were diagnosed with dementia.
Compared with subjects whose spouse did not develop dementia, those married to individuals with dementia had a six times increased risk of developing dementia.
What might explain the link?
The stress of looking after those with dementia may in part explain the risk, the researchers believe.
'The chronic and often severe stress associated with dementia caregiving may exert substantial risk for the development of dementia in spouse caregivers,' they said.
If caregiver stress does play a role, studies should examine whether interventions can be developed for those most at risk, the researchers argue.
Have other studies suggested a similar link?
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said this was the first time that having a spouse with dementia has been suggested to increase dementia risk.
'Previous studies have observed a link between the stress experienced by dementia carers and the onset of health problems, such as depression,' she said. 'We are also unsure of the link with any environmental factors.'
Other studies of the effect of relationships suggest that being married reduces people's risk of developing dementia, she said.
'Scandinavian researchers have found that being married in old age generally reduces dementia risk, which may be because of the greater social interaction that couples experience with each other.'
Further research is therefore needed to verify the findings of this latest study, she believes. It could discern the extent of any spouse effect and how interventions might combat any greater dementia risk, she said.