It depends on the colour, I have quite a few of these and most are 100% cotton but one is not (my Heather Navy is 50% Cotton 50% Polyester). Here is some info from another website that sells them:
Sport Grey 90% Cotton 10% Polyester
Ash Grey 99% Cotton 1% Polyester
Dark Heather 50% Cotton 50% Polyester Safety Green 50% Cotton 50% Polyester"
It would be good if the product description on Amazon mentioned this, but most are 100% cotton.