Abstract: A radiation therapy skin marking device which may be used to delineate a radiation therapy portal area on a patient's skin surface. The device includes a set of radiation therapy skin markers which are releasably attached to a backing liner and include an adhesive surface such that the markers may be releasably attached to a patient's skin surface. The markers include printed lines so as to facilitate radiation therapy treatments by more obviously defining a radiation therapy portal area. The markers include various shapes including, for example, a marker of substantially circular shape with a ninety degree wedge cut therefrom such that the marker may be used to outline a ninety degree corner and a marker of substantially circular shape with a circular cut-out center such that the marker is defined by a circular outer edge and a circular inner edge both of which circular edges share a common center point.