The Cherhill White Horse, cut into the Marlborough Downs, is owned and maintained by the village of Cherhill.
The 18th Century landmark underwent a major facelift in 2002 after losing its whiteness and shape, but now requires a "re-chalking" every two years.
David Grafton, from Cherhill, said 14 tons of chalk would be needed to "groom the horse" and restore it.
The horse is the second oldest in Wiltshire and one of nine in the county.
To combat discolouring from the weather, it used to be continuously scraped to reveal fresh chalk.
Now, to avoid the figure sinking below the surrounding ground, the ancient monument is topped up with extra chalk.
"The chalk had been bagged up and transported up the hill, ready to be slid down the bank to the horse," said Mr Grafton.
"After the surface of the horse has been cleaned of weeds and the outline tidied, the chalk will be spread on the cleaned surface. Obviously, the work will not do itself so additional volunteers are welcome to come and help."
The re-chalking is taking place from 10:00 BST and is expected to take up to six hours.
More than 40 volunteers are taking part in re-chalking a 232-year-old hill figure in Wiltshire.