To make barbed wire in Bloxburg, use the Barbed Wire Fence item in Build Mode. It was added in Version 0.8.7 and costs $33.

Quick steps

  1. Enter Build Mode.
  2. Open the fence section.
  3. Select Barbed Wire Fence.
  4. Place it where you want it, then adjust the layout as needed.

Note

If you were asking for a custom-looking barbed wire effect, players sometimes build a fake version with other fence pieces or decorations, but the actual in-game item is the Barbed Wire Fence.

Item info

Item| Price| Source
---|---|---
Barbed Wire Fence| $33| 1

TL;DR: In Bloxburg, you just buy the Barbed Wire Fence from Build Mode for $33 and place it like a normal fence.