Although polyploidy is rampant in ferns and plays a major role in shaping their diversity, the evolutionary history of many polyploid species remains poorly understood. Nuclear DNA sequences can provide valuable information for identifying polyploid origins; however, remarkably few nuclear markers have been developed specifically for ferns, and previously published primer sets do not work well in many fern lineages. In this study, we present new primer sequences for the amplification of a portion of the nuclear gapCp gene (encoding a glyceraldehyde-3-phosphate dehydrogenase). Through a broad survey across ferns, we demonstrate that these primers are nearly universal for this clade. With a case study in cheilanthoids, we show that this rapidly evolving marker is a powerful tool for discriminating between autopolyploids and allopolyploids. Our results indicate that gapCp holds considerable potential for addressing species-level questions across the fern tree of life.