This shirt is definitely authentic ! Also this is a very high grade shirt the pictures don't really do this any justice it's much nicer then the pictures appear. Super lightweight very good venting itch free stitching with a cool little pocket for a key or energy gel ect down on the bottom the material is like a micro waffle pattern fabric . Perfect warm weather running shirt. Im 6' 187 lbs I went for the medium for running the medium is a taylored fit on me if I were getting this for casual use I'd probably try a large for a little more wiggle room but for running medium is perfect.