Cosmo's Den is a members-only land den that was formerly sold at the Diamond Shop.
On the inside, both floors have small cracks filled with flowing water. There are also built-in wooden shelves on the walls. There is a staircase made of leaves between floors. The wallpaper usually turns plant-themed. The flooring turns into carpets that are placed in various different sections of the den. There is a small stream flowing from upstairs to downstairs. Outside, there is a big field.
This den is featured in the Meet Cosmo adventure and appears to be part of a giant tree.
The flooring is shown in rugs and mats that cover the ground, instead of one continuous floor. This is the first known den to have this feature.