The technique was successfully tested with 90 per cent accuracy on a group of 74 US war veterans.
It is the first time any objective test for the disorder has been found to work. Conventional brain scans cannot show if a person has PTSD.
The condition is caused by exposure to psychologically traumatic events occurring in war or as a result of accidents, natural disasters or violent crime.
It gives rise to symptoms such as flashbacks, recurring nightmares, angry moods, or "jumpiness" that can last for many years.
The US veterans, all diagnosed as suffering from PTSD, had served either in the Second World War, Vietnam, Afghanistan or Iraq.
The test employs a technique called magnetoencephalography (MEG) which analyses magnetic charges released by synchronous connections between populations of brain cells.
Researchers used MEG to distinguish between the veterans and 250 healthy members of the public not affected by PTSD.