The accident happened on the main road N15 that goes through the Barnesmore Gap, in an area known locally as McGroary's Brae.
Gardaí are advising motorists wishing to travel to South Donegal to divert via Glenties, and if possible to avoid the area completely.
Significant delays have been reported along the diversion routes, while traffic is particularly bad around Stranorlar and Ballybofey.
It has been reported that the motorcyclist was a marshall providing support for the cyclists taking part in the Cycle Against Suicide.
According to reports the cyclists have turned back to Donegal Town as they decide what to do following the fatal collision
The cyclists were due to arrive in Derry this evening.
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