Polar fleece, usually referred to simply as "fleece," is a soft napped insulating synthetic fabric made from Polyethylene terephthalate (PET) or other synthetic fibers. One of the first forms was Polar Fleece created in 1979 by Malden Mills, now Polartec LLC., a new, light and strong pile fabric meant to mimic and in some ways surpass wool. Fleece has some of wool's finest qualities but weighs a fraction of the lightest available woolens.

Polar fleece is used in casual jackets, hats, sweaters, jogging bottoms/sweatpants, gym clothes, hoodies, inexpensive throw blankets, and high-performance outdoor clothing, and can be a vegan alternative to wool. It can be made partially from recycled plastic bottles and is very light, soft and easy to wash.

Aaron Feuerstein intentionally declined to patent Polar fleece, allowing the material to be produced cheaply and widely by many vendors, promoting the material's wide acceptance.

Fleece garments traditionally come in different thickness: micro, 100, 200, and 300, with 300 being the thickest and least flexible.