The global threat posed by invasive alien plants has prompted inventory compilations and screening exercises that aim to understand invasiveness in various taxa. Various traits influence the invasiveness of a species but do not apply to all plant taxa. Ferns are rare or absent from such inventories, but notable fern invasions do exist. We developed a global inventory of terrestrial alien true ferns (Polypodiophyta) comprising 157 species, using published literature and online inventories. We aimed to determine which traits influence the probability that a terrestrial alien fern will become naturalised or invasive. Generalised linear models with transition stages as response variables, were used to assess the effects of various anthropogenic, biological and distributional traits on invasiveness. Our model explained 30–40% of the variance associated with invasiveness and showed that a ground-dwelling life form, reproductive plasticity, tolerance for disturbance and varied light conditions, and a broad introduced range (interpreted as high environmental tolerance and popularity in horticulture) were important determinants of invasiveness in alien ferns. We highlighted which geographic regions and fern families had the highest incidences of alien ferns and identified particular species of concern. This study aids in the understanding of the mechanisms underlying invasiveness in alien ferns and the findings can inform future research on this understudied taxon as invasive species.