Milk will replace sugar as the treat's main ingredient, as the white stuff content is boosted from 26%  to 37.5%.
Nestle said the new recipe bars - with sugar cut by 0.7g - will be in stores by the end of the month.
It follows a similar move with its KitKat recipe, which increased the milk and cocoa content.
The Milkybar recipe has been the same since the bar first came into production in 1936, but is being changed as part of Public Health England's drive to reduce the amount of sugar consumed in the UK by 20%.
In April Nestle reduced the amount of sugar in its fruit pastilles by 30%.
A spokesman said the reductions in sugar would make "a positive step overall to removing sugar from the nation's diet while maintaining the taste"
Milkybars are about to get milkier, as the recipe changes for the first time in 81 years.