The Ad hoc checklist of alien species in Belgium is a species checklist dataset published by the Research Institute of Nature and Forest (INBO). It was specifically created for the TrIAS project (Tracking Invasive Alien Species, http://trias-project.be) to account for gaps in the alien species coverage in other species checklists for Belgium and mainly includes taxonomic groups and newly introduced species not covered elsewhere (yet). Due to its ad hoc nature, the list might change substantially over time. Here it is published as a standardized Darwin Core Archive and includes for each species: the scientific name, higher classification and stable taxon identifier (in the taxon core), the (confidence regarding the) presence of the species in Belgium (and its regions), the date of first introduction and last assessment (in the distribution extension), coarse habitat information (in the species profile extension), the pathway(s) of introduction, native range(s) and invasion stage in Belgium (in the description extension), and an overview of the consulted literature for each taxon (in the literature references extension). Issues with the dataset can be reported at https://github.com/trias-project/ad-hoc-checklist We have released this dataset to the public domain under a Creative Commons Zero waiver. We would appreciate it if you follow the INBO norms for data use (https://www.inbo.be/en/norms-data-use) when using the data. If you have any questions regarding this dataset, don't hesitate to contact us via the contact information provided in the metadata or via opendata@inbo.be. This dataset was published as open data for the TrIAS project (Tracking Invasive Alien Species http://trias-project.be, Vanderhoeven et al. 2017), with technical support provided by the Research Institute for Nature and Forest (INBO). It is selected as one of the authoritative sources for the compilation of a unified and reproducible checklist of alien species in Belgium.