The Alzheimer's Society says two in three people with dementia are not diagnosed
The number of people in Wales with dementia is set to rise by around 11,000 over the next 13 years, claims the Alzheimer's Society.
The charity says the number of people with the condition will rise from 37,000 to 48,000 by 2021.
The society is launching a campaign to make people understand the need to seek medical help if someone's forgetfulness is a cause of concern.
Alzheimer's disease is the most common of more than 100 types of dementia.
The Alzheimer's Society's campaign, Worried About Your Memory?, aims to prompt people to think whether their forgetfulness, or that of a friend or relative is just due to poor memory or the start of a medical problem.
Symptoms of dementia include memory loss, confusion and problems with speech and understanding.
Alzheimer's disease affects 62% of those diagnosed in the UK, while other forms include vascular dementia and mixed dementia.
The Alzheimer's Society, which has 21 branches in Wales, said up to two thirds of people with dementia were not diagnosed because they either do not recognise or report their symptoms.
It said its research had shown it can take an average of nearly three years for dementia to be diagnosed from the time when the first symptoms are noticed.
Research by London School of Economics and King's College London commissioned by the Alzheimer's Society found Cardiff and Swansea have the highest number of people with dementia, while Ceredigion and Powys will see some of the biggest growth in the condition.
The campaign will also send a CD ROM to help diagnosis to every GP in Wales.
Dr Tony Bayer, director of the Cardiff Memory Team at University Hospital Llandough, said: "Identifying the symptoms of dementia early is crucial in order to trigger prompt and appropriate health and social care support."
Ian Thomas, director of Alzheimer's Society in Wales, said: "Dementia is one of the greatest health challenges confronting our society as we face an ageing population.
"Yet there is an acute lack of awareness and understanding of dementia amongst the public, including those who are affected by it, as well as those in health and community services."