The warning came into force at midnight and will remain in place until midday.
It covers the area from Aviemore to Banff, and reaches to Elgin in the north east.
The Met Office has warned of surface spray on the roads and potential flooding which may cause disruption.
Traffic Scotland has said people should prepare for longer journey times, with bus and train services at risk of delays.
There is a chance that homes and businesses could become flooded as a result of the heavy and persistent rain.
The rest of the country is also forecast to experience wet weather, though the warning hasn’t been extended, and drivers have been advised to travel with care.
On top of the rain, blustery winds are expected, particularly in the west.
Tomorrow will brighten up slightly with sunshine forecast between showers.