A polygon unit-based image processing method, and a device for the same are disclosed. Specifically, a method for decoding an image on the basis of a polygon unit can comprise the steps of: deriving a motion vector predictor for a polygon apex forming the polygon unit; deriving a motion vector for the polygon apex on the basis of a motion vector difference for the polygon apex and the motion vector predictor; and deriving a prediction sample for the polygon unit from a division unit, which is specified by the motion vector, in a reference picture.