Abstract: A floor mat retainer for securing a floor mat to mass backed carpeting of the type having an upper carpet layer bonded to a lower carpet layer. The retainer includes a self-piercing shank having a high lead tapered thread terminating at a lower partial radial shoulder. An upper radial shoulder is spaced from the lower shoulder by a distance less than the distance of the carpet layer. The retainer is self-piercingly threaded into the carpeting until the lower shoulder moves through the carpet layer to thereby press the carpet layer between both shoulders. The length of the shank is less than the distance between the vehicle floor and compressed carpet layer to avoid bulging. A headed portion extends from the upper shoulder and is received through an opening in the floor mat to retain the floor mat in place. The retainer is of one piece molded plastic construction to eliminate rust and corrosion problems.