Researchers from University College London and Cardiff University examined the brains of five people who had been professional footballers and one who had been a committed amateur throughout his life.
They had played football for an average of 26 years and all six went on to develop dementia in their 60s.
While performing post mortem examinations, scientists found signs of brain injury - called chronic traumatic encephalopathy (CTE) in four cases.
CTE has been linked to memory loss, depression and dementia and has been seen in other contact sports.
The research follows anecdotal reports that players who head balls may be more prone to developing dementia later in life.
The Football Association says it will look at this area more closely.
Experts said recreational players were unlikely to incur problems.