Richard Johns, 52, set out alone from Halls Gap to Barooka Lookout in the Grampians National Park around 2pm yesterday and never returned.
Johns is not familiar with the area, Victoria Police said in a statement.
Airborne police officers, park rangers and SES have been scouring the bush for any sign of the missing hiker.
The temperature is forecast to drop to 1C tonight.
Johns was last seen wearing a blue zip-up jacket and pants.
Police urge anyone with information on his whereabouts to contact the Ararat Police Station on 53355 1500.