— Game Guide(src)
Mighty Belts are a type of class-specific armor for the Barbarians in Diablo III. These are heavier, fearsome Belts, often so large that they are essentially an extra piece of chest armor. Many of these belts are reinforced with metal plates and decorated with skulls of the fallen foes instead of belt buckles.
For gameplay purposes, Mighty Belts are the only type of class-specific armor to offer extra defense compared to normal counterparts, giving Barbarians additional fighting chances in melee combat. In addition to the stats normally allowed on regular belts, a Mighty Belt may also roll maximum Fury and Life per Fury spent. Despite their massive size, they are not displayed on a character model.