Method 1: Crafting Table
Place 3 Prismarine in a Crafting Table to make 6 Prismarine Slabs.
6 How to Make a Prismarine Slab in Minecraft
Use this simple recipe to make a Prismarine Slab in Minecraft. You can see the exact ingredients, recipe layout, and workstation below.
Prismarine Slab recipe
Follow the recipe below. The pattern must use the correct slots when the recipe is shaped.
6 
Method 2: Stonecutter
Use 1 Prismarine in a Stonecutter to make 2 Prismarine Slabs.
2 About this Minecraft recipe
This page shows the vanilla recipe for Prismarine Slab. It includes the exact ingredients, recipe layout, required workstation, and amount produced.
The data comes from Minecraft Java 26.2. Recipes can change in later game versions, so check the version you play before crafting.