We report the discovery of the largest fern genome in Tmesipteris obliqua (1C = 150.61 pg), a species belonging to the whisk-fern family, Psilotaceae. Its genome size estimated by flow cytometry is comparable with the largest genome so far reported for any eukaryote, in the monocot Paris japonica (Melanthiaceae; 1C = 152.23 pg). The addition of this new record doubles the range of genome size values so far encountered in ferns from 97.2-fold (0.77-74.84 pg/1C) to 196-fold (0.77-150.61 pg/1C). This finding emphasizes the importance of filling taxonomic gaps in our knowledge to uncover the full extent of genome size diversity across the different lineages of land plants.