Ambulances are at the scene of the incident which took place near the Raphoe junction turn-off.
Gardai have asked motorists to avoid the area as an investigation gets underway.
It’s understood a vehicle left the road close to where the GoSafe speed vans are often located.
The incident came on a bank holiday weekend when Gardai had issued a public appeal for safe driving in Co Donegal.
A Garda spokesman said: “The road is closed due to a serious road traffic accident and it will remain closed for some considerable time. Investigations are under way.”