Crab helmet is a type of helmet, a type of equipment which provides the same protection as a diamond helmet. In addition, while wearing it, crabs and King Crab will follow you and won't attack you. Agressive crabs do not automatically calm down by following you, although you can use a crab spear to manually calm them down. In conjunction with the crab spear, it can be used to command crabs. With the crab spear, this item on, and on a king crab, the crab spear can do as much damage as a lava spear.
While equipping this item, you can't damage crabs with anything. However, you can aggrivate crabs by "hitting them". Also, if you use a saddle on a king crab while wearing a crab helmet, you have a chance to tame the king crab.
This item was added in the "Crab in the Bed" update.