Biological invasions are a serious threat globally, but particularly in developing countries. Bhutan is unique among South Asian countries in that it has a rich biodiversity, and its people have a ‘sacred’ responsibility to protect the environment and native biodiversity; conversely it is also considered a ‘crisis ecoregion’ because of significant threats to biodiversity from anthropocentric activities. Managing biological invasions is difficult without a comprehensive baseline of the alien species present. An alien plant inventory for Bhutan was created by examining an extensive array of information and data sources such as herbaria records, published floras, unpublished documents, and from personal communications. The alien plant flora including cultivated taxa in Bhutan comprises 139 families, 545 genera and 964 species. Of these, 626 species occur only in cultivation, whilst the other 338 species occur in the wild (spontaneous); 131 (39%) casuals, 103 (31%) naturalised and 101 (30%) invasive. The major of naturalised alien plants were introduced as pasture species (32%), ornamentals (24%) and from unintentional sources (22%). Whilst, the major of invasive species were introduced unintentionally (76%), as ornamentals (15%) and pasture species (3%). Because a large proportion of alien plants have been deliberately introduced, implementation of both pre-border weed risk assessment and post-border weed risk management approaches can be effective in Bhutan, despite the country’s open and porous borders. Such a biosecurity approach could also be implemented on a plant import sector basis, as only four sectors account for 86% of alien plant introductions, largely through one entry point. The baseline inventory and analysis will shape future management and policy directions for alien plants in Bhutan.