The YouGov survey showed 31% of people questioned would not know how to get information and support if they were diagnosed with the condition.
However, 82% of people said they would turn to a professional for help.
Ruth Sutherland, acting chief executive of the Alzheimer’s Society, said it was ‘terrible’ that so many people would not know what to do if diagnosed with the condition.
‘A million people will develop the condition in the next 10 years. We need better awareness and information so that a diagnosis of dementia is not such a frightening experience,’ she said.
The survey also found that 14% of people would want support but would have no one to turn to if diagnosed with dementia.
Ms Sutherland added: ‘We must tackle the stigma surrounding the condition if we are to ensure people with dementia are treated with the dignity and respect they deserve.’
Her comments came as Dementia Awareness Week was launched on Sunday.
A third of people will die with dementia, a condition that is estimated to cost England £15.9bn each year.