Figures from the NHS show 2,963 people aged 65 and older in Barnsley were estimated to have dementia in March 2023.
Of them, 2,076 had a formal diagnosis.
It meant 29.9 per cent of people believed to have dementia in the area did not have a diagnosis - more than a quarter.
The number of residents being diagnosed with the illness has risen.
Data from the Office for Health Improvement and Disparities shows just 64.5 per cent of people thought to have dementia in the NHS Barnsley CCG area were formally diagnosed in 2021 - based on an indicator that predicts the expected number of dementia cases in people aged 65 and over.
Paul Edwards, director of clinical services at Dementia UK, said dementia is a growing issue in society largely due to an ageing population.
Mr Edwards added continued cuts and a lack of focus on community services which support families with dementia have led to ‘thousands being stuck in limbo and highly stressed’ as they are unable to get a diagnosis.
“In this climate of stretched services and increasing diagnosis rates, we urgently need to bridge the gap between health and social care systems, so that people receive a timely diagnosis and can access the support they need,” he said.
“No one should have to face dementia alone.”
Across England, an estimated 687,000 people aged 65 and older have dementia.
Of them, 433,000 patients had a recorded diagnosis of dementia as of March this year.
Mark MacDonald, Alzheimer’s Society associate director for advocacy, said dementia diagnosis rates hit a five-year low during the pandemic and have stagnated ever since.
He added: “This means that tens of thousands of people are facing the realities of dementia alone, without access to the vital care and support that a diagnosis can bring.
“Timely diagnosis is crucial to manage symptoms and avoid ending up in crisis.
“Getting a diagnosis can be daunting, but we believe it’s better to know.
“Alzheimer’s Society is encouraging those who are concerned that they or a someone close to them may be experiencing signs of dementia to check their symptoms on our Royal College of GPs-accredited symptom checker.”
A Department of Health and Social Care spokesperson said: “Timely diagnosis of dementia is vital, and the NHS is committed to bringing the dementia diagnosis rate back to pre-pandemic levels.
“Last week, the Health and Social Care Secretary also signed an agreement at the G7 Health Minsters’ meeting to commit to working with other nations to help tackle health issues such as dementia on a global scale.”