A survey of 1,043 people showed 41 per cent "bottled up" their concerns about dementia, mainly because they feared upsetting the person affected or they did not want to face the reality of the condition.
The study for the Alzheimer's Society, carried out on Facebook and online, showed 54 per cent had waited for six months or more after first noticing symptoms of dementia in themselves or someone close to them before seeking professional help.
The charity said an estimated 52 per cent of the 800,000 people living with dementia in the UK are yet to receive a diagnosis. The high numbers who are failing to talk about the condition has added to these difficulties, the charity said.
A separate online survey of 2,358 adults found dementia is the condition people are most reluctant to seek help for, compared to other health issues such as heart and digestive problems.
The YouGov poll also found 57 per cent of adults who have had a health problem in the past year have put off seeking help about it, nearly half of them because they thought they would be "making a fuss".
Nearly a quarter (23 per cent) of those who put off seeking help said it was because they feared it was a serious health problem, such as dementia. The findings have been released in Dementia Awareness Week.
Jeremy Hughes, Alzheimer's Society chief executive, said: "One in three people over the age of 65 will develop dementia, so it is worrying that so many people feel reluctant to seek help about it."
The survey findings come after musicians Chris Martin, Lily Allen and Alesha Dixon joined other celebrities in a new TV advert at encouraging people to befriend people with dementia. Author Sir Terry Pratchett - who suffers from Alzheimer's disease himself - is also involved.