A pre-assembled anti-creep waist-clothing stay device including a patch having front and back surfaces and sized for application to a crotch-adjacent inner-seam area of the waist-clothing and a stay strip affixed to the back surface of the patch, the stay strip being positioned and sized such that the patch extends beyond edges of the stay strip. In key embodiments, the patch includes a heat-activated adhesive such that, when the patch is applied to the waist-clothing with iron-on heat and pressure, the patch adheres at its back surface to the waist-clothing, thereby securing the stay strip in position to prevent the waist-clothing from riding up and bunching in the crotch area.