Gardai have warned motorists to slow down with traffic officers already stopped vehicles in the Letterkenny area for speeding through the fog.
There will also be icy stretches on many roads, particularly inland and in hilly areas, said Met Eireann.
It upgraded a previous Yellow Alert to Orange status a short time ago.
The alert remains in place until 9am tomorrow.
A Garda spokesman said: “Despite very thick fog in many parts of Donegal over the past few hours, some motorists have continued to drive with undue care and attention.
“Patrols are out this evening to ensure the safety of all motorists. Drivers need to slow down, used dipped headlights and rear fog lights in conditions like this.”