Shell blocks are blocks that appear in Mario & Luigi: Bowser's Inside Story and its remake. Striking one allows Mario to wear a Blue Shell, and Luigi can then knock him forward using his hammer. This block is made available when Mario and Luigi meet the Koopa Paratroopa Trio. During their travels, Mario and Luigi find a giant shell block above the clouds in west Toad Town and strike it, spreading several normal-sized shell blocks around the Mushroom Kingdom. The Koopa Paratroopa Trio notices this and begins yelling at the two. Eventually, the trio gives the Mario Bros. a block to use. Mario strikes the block, and a shell comes out and lands on his back. After he uses it, the Koopa Paratroopa Trio sees that the Mario Bros. can use the shell quite well and teaches them the proper way to execute it. Once they learn correctly, the Koopa Paratroopa Trio flies off and gives the two the ten Toad Town Attack Pieces for the Yoo Who Cannon. The blocks are scattered around the overworld and inside Bowser for Mario and Luigi to use.
If the player attacks an enemy with the shell power-up in the field, the enemy will be temporarily stunned.
- The shell that goes on Mario is somewhat similar to the shell Kooper wears in Paper Mario, although it is more likely to be based more on the Shell Mario power-up from New Super Mario Bros.