Frequent showers will increasingly turn to snow on high ground and inland from 18:00 GMT on Thursday, with sleet and snow heading south-eastwards across Wales.
Up to 4cm (1.6in) of snow could fall at low levels, with up to 10cm (4in) on higher ground.
Snow has already caused problems for motorists on higher ground.
The Met Office urged people to be aware of the likelihood of "some travel disruption and difficult driving conditions", with the warning covering the period up to 11:00 on Friday.
In Denbighshire, the A542 Horseshoe Pass was closed at Llangollen but has since reopened. The A494 near Ruthin and Mold has also been affected.
A lorry, which was stuck in the snow at nearby Clwyd Gate has since been moved, said North Wales Police.
South Wales Fire and Rescue Service Fire Service reported several weather-related accidents on Thursday morning.
In the Swansea Valley, a car overturned in icy conditions, closing the A474 at Rhyd y Fro near Pontardawe.
Meanwhile, flood water has closed the A548 coast road at Mostyn, Flintshire.
Check if this is affecting your journey
A yellow "be aware" warning of snow and ice across Wales has been issued by the Met Office.