The checklist of alien mammals of Belgium is a species checklist dataset published by the Research Institute for Nature and Forest (INBO). It contains information on alien mammal species recorded in the wild in Belgium since 1976. Both established species and occasional records (casuals) are included, but with information on the degree of establishment of species following the unified invasion framework of Blackburn et al. (2011). The checklist is published here as a standardized Darwin Core Archive and includes for each (sub)species: the scientific name and classification (in the taxon core), the presence in Flanders, Wallonia and the Brussels-Capital Region, year of first and last observation in Belgium, the degree of establishment and pathway(s) of introduction (in the distribution extension), coarse habitat information (in the species profile extension), and native range(s) (in the description extension). A reference (scientific, gray literature, online source) to this information is provided. Issues with the dataset can be reported at https://github.com/trias-project/alien-mammals-checklist/issues. We have released this dataset to the public domain under a Creative Commons Zero waiver. We would appreciate it if you follow the GBIF citation guidelines (https://www.gbif.org/citation-guidelines) 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. 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 (GRIIS Belgium, Desmet et al. 2023).