Soft Fleece - 100% Polyester
The same fabric used in making baby ear-warmers and bedsheets. That should tell you how cozy it is. It is widely used in the garment industry, here's why:

Strong, durable, yet lightweight.
Retains heat and keeps warmth.
Retains shape.
Wrinkle-resistant.

Note: size 2XL and 3XL does not have D-rings on the back for safety reasons.